The relevant bit of the Pathfinder Society Field Guide:
At character creation and each time you gain a level, you decide if
you would like your character to be on the standard or slow
advancement track for the entirety of that level.
Primarily, the "slow" XP gain is so that you can keep playing with a particular character longer. Note that when you gain a level you pick whether you are on the slow or normal track for the next level. There are a couple of reasons you might do this:
Hypothetical 1: I like playing PFS with my friends, but I attend more games than they do
I might decide to do ½ XP for a few levels, so that I'll stay in the same tier with them.
Hypothetical 2: I really love this character, but I'm approaching the level 12 cap too rapidly!
I might then do ½ XP to prolong the life of that character.
Hypothetical 3: I GM a lot of PFS games, and all of my characters are getting XP, but I've not had time to play them
Might choose ½ XP for a level or so, in order to actually get to play the character at that level.
In the end, it's just a way to stretch out the pacing of PFS games, since there's not a lot of end-game content at level 12, and if you play a lot, you'll be leveling up fairly quickly.
http://paizo.com/pathfinderSociety/resources
Gunsmithing does not grant the ability to craft firearms, ammunition, or black powder. Rather, it allows the purchase of bullets, pellets, black powder, and alchemical cartridges (with 1 rank in Craft [alchemy]) at the listed reduced price, but does not grant a discount on the purchase of any firearm. Resold items gained through this feat are worth half the actual cost paid, not half the regular market value for the item. No PC can purchase a gun without this feat, even if they possess the Amateur Gunslinger or Exotic Weapon Proficiency (firearm) feats.
The part that specifically answers your question is in bold
Best Answer
It's all the same system - you get a set amount of gold at the end of the game, which is an abstract representation of how much gold you'd have if you sold everything and split up the proceeds, plus any actual cash or rewards you might be getting. It's already got your #1 - #3 covered for you.
You can then use that gold to purchase "stuff" - whether it's loot from that game's Chronicle Sheet, loot off one of your older Chronicle Sheets, basic gear from the core books, or equipment from the additional resources list (subject to the rules on that page).
One note: anything you find during a scenario can be used for free within that scenario (including single-use items), but you have to purchase it if you want it for use in future scenarios. You do not have to pay for potions and scrolls found and used during a scenario out of your gold for that scenario. (See p.19 of the Guide.)