As Flenyar quoted in his answer:
Each time he gains a new level, he chooses two classes, takes the best
aspects of each and applies them to his characteristic.
This implies that if one class would gain a BAB at it's respective level, you do as well. Note that unlike normal multiclass characters, your stats do depend on what order you take the classes in.
Notice how Flenyar's progression (6 levels of Fighter/Rogue, then 4 levels of Wizard/Rogue) gives a BAB of +9, but the following progression gives you a full BAB of +10, even though the character is still a Fighter 6/Wizard 4//Rogue 10:
- Fighter 1 / Rogue 1 (+1 [Fighters gain +1 every level])
- Fighter 2 / Rogue 2 (+1 [Fighters gain +1 every level])
- Fighter 3 / Rogue 3 (+1 [Fighters gain +1 every level])
- Fighter 4 / Rogue 4 (+1 [Fighters gain +1 every level])
- Fighter 5 / Rogue 5 (+1 [Fighters gain +1 every level])
- Wizard 1 / Rogue 6 (+1 [Rogues gain +1 at 6th level])
- Wizard 2 / Rogue 7 (+1 [Both Wizards and Rogues BAB improves])
- Wizard 3 / Rogue 8 (+1 [Rogues gain +1 at 8th level])
- Fighter 6 / Rogue 9 (+1 [Fighters gain +1 every level])
- Wizard 4 / Rogue 10 (+1 [Both Wizards and Rogues BAB improves])
Total BAB at level 10 is +10.
This means that even a Wizard//Sorcerer gestalt (or any other two low BAB classes) can have a perfect BAB if you take just one level of another perfect BAB class:
- Wizard 1 / Fighter 1 (+1 [Fighters gain +1 every level])
- Wizard 2 / Sorcerer 1 (+1 [Wizards gain +1 at 2nd level])
- Wizard 3 / Sorcerer 2 (+1 [Sorcerer gain +1 at 2nd level])
- Wizard 4 / Sorcerer 3 (+1 [Wizards gain +1 at 4th level])
- Wizard 5 / Sorcerer 4 (+1 [Sorcerer gain +1 at 4th level])
- Wizard 6 / Sorcerer 5 (+1 [Wizards gain +1 at 6th level])
- Wizard 7 / Sorcerer 6 (+1 [Sorcerer gain +1 at 6th level])
- Wizard 8 / Sorcerer 7 (+1 [Wizards gain +1 at 8th level])
- Wizard 9 / Sorcerer 8 (+1 [Sorcerer gain +1 at 8th level])
- Wizard 10 / Sorcerer 9 (+1 [Wizards gain +1 at 10th level])
Total BAB at level 10 for this Wizard 10//Fighter 1/Sorcerer 9 is +10 (perfect.) This cannot be determined without the order of the levels taken.
...the DM may overrule this, based on the wording of this rule:
Base Attack Bonus: Choose the better progression from the two classes. (Emphasis mine.)
It is conceivable, then, that the Wizard 10//Fighter 1/Sorcerer 9 is a character with 1 level of perfect BAB progression (+1) and 9 levels of low BAB progression (+4), leaving him with only a BAB of +5.
I'm not sure of any official errata on the matter, but it makes a big difference in some cases. Ask your DM, or make sure your players know your ruling if you are the DM. Personally, I would stick with the latter "better progression" rule over the "better increase" rule, since it seems to be the intent of gestalt.
There is also the "Fractional Base Bonuses" house rule, presented on Unearthed Arcana p.73, which is designed to allow smooth leveling of gestalt multiclass characters without any of these exploitable loopholes.
Since you mentioned saves in your comment, I'll touch on it briefly:
Saves would indeed work the same was as BAB, but beware that they are even easier to inflate artificially if you use the first presented "better increase" rule. This is because at first level of every class with good saves, that save "increases" from +0 to +2. Therefore it's even easier to end up with ridiculously high saves through multiclassing gestalt, if you don't simple lump all progression levels together before calculating character stats.
As a side note, consider two classes which gain 1d6 sneak attack every other level. Staggering them as I suggest staggering BAB increases still cannot double your sneak attack damage, since the book explicitly states:
Class features that two classes share accrue at the rate of the faster class.
Personally, I’d say you’re about done with fighter and swashbuckler
I try to avoid taking more than 3 levels of swashbuckler (insightful strike is its best feature) or 2 levels of fighter (a feat per level is OK; a feat every other level is pretty weak). I’d say it’s probably a good time to leave those classes behind.
The exception is if you take, e.g. Daring Outlaw (see below) to improve these classes. The Zhentarim Soldier ACF can also do wonders for the fighter class (up to 9th level anyway) if you have the Charisma and Intimidate ranks for it.
The best answer is warblade (but you don’t have that allowed)
Just for the sake of anyone else reading this, the warblade (Tome of Battle) is perfect here: he’s flashy, highly-skilled, and makes good use of Intelligence (and he gets 4+Int skills). White Raven makes him an excellent battlefield leader.
You already have Int to damage: factotum adds Int to (almost) everything else
(I just noticed you don’t have factotum listed either; that’s a real shame. Again, leaving this here for future users who do)
Factotum (Dungeonscape) sounds really perfect for your character here. They do gain a few spells (and some Supernatural abilities), though there are homebrew variants that can remove them. More importantly, the factotum’s whole schtick is “I add Intelligence to everything.” Like all Strength and Dexterity skill and ability checks (e.g. Athletics, Acrobatics, Stealth, Initiative) at level 3. In fact, the only notable thing that factota don’t add Intelligence to is damage: that you already have from swashbuckler’s insightful strike.
And a factotum is always the man with the plan, even if he’s actually just making it all up as he goes along. Seems like a pretty good guy to follow.
See this question for more information on using the factotum class well.
You may already qualify for chameleon, and it’s pretty good
You may qualify for the chameleon prestige class (Heroes of Destiny or here) based on Able Learner (depends on whether or not you’ve been putting points in the skills you need). While the best feature of chameleon is their spellcasting options, you don’t need to use them. Two levels of chameleon gets you the wonderful “floating feat,” a feat you can change every day. A third level can get you some sneak attack damage, which is useful (see below re: Daring Outlaw).
If you qualify for that, you might also make a good master of masks
This prestige class (Complete Scoundrel or here) isn’t very good and is probably way too specific for you: you want to be a leader of men, not hiding behind constantly-changing masks. But if you take just one level (i.e. only get one mask), and choose the gladiator mask, you get to be Maximus. You become proficient in every weapon ever, which is pretty awesome, and we all know how Maximus leaves everyone entertained.
Dread commando is fitting, but not very strong
The dread commando prestige class (Heroes of Battle or here) is full-BAB and gets sudden strike, which is the weak version of sneak attack (and stacks with sneak attack for qualifying). Requires Dodge and Mobility, which is a shame since they’re pretty weak and you should have ranks in Tumble to avoid AoOs anyway. Mobility, at least, can be gotten as an armor special ability (and the rules do allow you to qualify using items, but make sure DM allows it), but it’s quite expensive for your level.
Note that there are several feats (Midnight Dodge from Magic of Incarnum, Expeditious Dodge from Races of the Wild, Desert Wind Dodge from Tome of Battle) that count as Dodge and are better. Still not good, but better. Worth considering.
Anyway, dread commandos are skilled fighters, and the bonus damage is good for a finesse build, and you get some team-based bonuses. So fitting. But it’s a weak form of bonus damage and the team bonuses are pretty small, so not very good.
A little bit of bard could work
Bards require ranks in Perform, but unless you use their spells they don’t actually need Charisma. Just one level for inspire courage would not be a bad idea for the sake of being all leader-y. You can just ignore the cantrips that you get.
See this question for thoughts on making the most of the bard class.
Knight is OK for a few levels
Knight (Player’s Handbook II or here) is heavily-armored and full-BAB, and makes some solid attempts to protect others (e.g. test of mettle). Unfortunately, the knight’s code is extremely strict, even stricter than the paladin’s in a lot of ways.
It’s often said that knight has five good levels: 1, 2, 3, 4, and 20. If you go for knight, test of mettle and bulwark of defense are probably your best bets.
If you can, refluff a barbarian
Instead of rage (or ferocity or whirling frenzy, which are probably more appropriate to you), call it something else: In the Zone, Focus, something. Refluff it as bullet time or something; the mechanics still work for that (particularly ferocity or whirling frenzy, which actually do make you move faster). One level in barbarian gets you these great features, and if you have Complete Champion available, you can also swap fast movement for pounce via the lion spirit totem alternate class feature, which is a great deal.
This doesn’t directly impact leadership skills, but it gives you a lot of martial prowess to back up your leadership.
Marshals are OK if you’ve got great Charisma
If you already have very-high Charisma (16-18 before items), marshal (Player’s Handbook II) is worth it for the minor auras that add that to various things. But since you can change your minor aura whenever you like, and the major auras are weirdly weak, I don't think you need more than one level.
Legendary leaders certainly fit the bill—if your DM is using the Commander rules
Legendary leader (Heroes of Battle) gives some solid abilities on an OK chassis. The problem is that it depends on the DM using the Commander rules found in Heroes of Battle, and in my experience, most don’t. Without them, a couple of abilities don’t really make sense.
If you do go this route, note that Iron Will can be bought for 3,000 gp (Otyugh Hole, Complete Scoundrel). Many DMs won’t allow it, but it’s definitely worth asking if you can. 3,000 gp is a bit much for a +2 bonus to a single save, but as something that counts for prerequisites, it’s a steal.
Daring Outlaw would improve your damage
The Daring Outlaw feat (Complete Scoundrel) stacks your swashbuckler and rogue levels for grace and sneak attack. You need 2d6 sneak attack damage to qualify, which means three levels in rogue, or a level in rogue and a level in something else that gives sneak attack (or an analogue like the ninja’s sudden strike or the scout’s skirmish; both from Complete Adventurer).
Note that a dread commando (above) could take Daring Outlaw because sudden strike stacks with and counts as sneak attack for prerequisites. This will give you +2d6 sneak attack dice from swashbuckler levels, which is nice, and allows you to consider going back to swashbuckler for more.
Daring Warrior isn’t too good
It stacks swashbuckler and fighter, two classes you already have, which makes it seem appropriate, but they only stack for the purposes of grace and qualifying for fighter-only feats. With the exception of Weapon Supremacy (Player’s Handbook II, requires Fighter 18), the fighter-only feats are pretty weak, and grace isn’t too great.
Knowledge Devotion is pretty solid
If you have Knowledge skills, it’s an attack and damage bonus. Just worth mentioning because it’s a good feat that might fit the character. Depends whether or not your wit is backed up by real knowledge, though, since Knowledge Devotion does require a fair amount of, ya know, devotion to knowledge.
A bunch of dips for useful abilities
I like Barbarian 1, Bard 1, Chameleon 2, and Master of Masks 1 best of the options presented that are on your list of legal material. I wouldn’t take more levels of any of these classes, however. That gets you to 10th-level, anyway. After that, I suppose dread commando and/or legendary leader are probably your best bets.
The simple option: Daring Outlaw and dread commando
The other option is to not bother with any of that hodgepodge and just go Swashbuckler 13/Fighter 2/Dread Commando 5 with Daring Outlaw. That gets you full BAB, 7d6 Sneak Attack, and 3d6 Sudden Strike. Of course, that assumes you can get Dodge and Mobility... Anyway, it will make you a reasonably effective melee combatant and give some minor bonuses to those around you. It’s not the best build available but it does have a certain elegance.
I really do have to mention warblade again
If you can get it allowed, I have to mention again how perfect warblade is here. With a start of swashbuckler 3/fighter 2, you can do really well by taking three other classes, and then taking warblade at 9th to start with an Initiator Level of 5, thereby starting with 3rd-level maneuvers and stances. My preferences for those three levels would be Factotum 3, Chameleon 2, Barbarian 1, Master of Masks 1, and/or Bard 1 (such that they add up to three, of course). For bard, Song of the White Raven becomes your obvious choice as 9th-level feat.
The two expressions are equal.
Every time the attack bonus reaches +6, you gain another attack at your full BAB -5 (that is: +1); but you must use the full attack action. You gain an additional attack at +11 (the additional attack is at your full BAB -10) and +16 (full BAB -15), independently by how you sum up our BAB (source).
Stating that a creature has a BAB +13, inherently implies that it has 3 attacks with the full attack action:
So, BAB +13 = BAB +13/+8/+3.