I been playing GURPS since 1987 and the basic combat of attack, defend, damage can indeed bog down for even moderately skilled players. In reality people have options to get around the defenses of a skilled opponent and for some added complexity GURPS can emulates these options.
First off the players should not pair off with their opponents but should at least double team opponents. In the two-on-two fight this means that one player can force a step back by an enemy then the second player can strike, forcing their opponent to use their base defense.
They could position themselves so one can limit the target's defense (page 390). A side attack get -2 to defense and a rear attack permits no defense roll.
The player can choose to beat down their opponents' shields by striking at their weapons and especially the shield (pages 484, 485).
A skilled player can chose to Feint, basically a contest of weapon skill, and if the player wins the contest the difference is applied as a negative to the opponent's defenses (page 365).
A limb strike to the arm or leg can be effective as there is often little armor in those locations (page 398).
A step and evaluate can be used to gain up to +3 to hit (page 364). While it extends combat in-game time, it takes little time to resolve in table time: the player just declares he is evaluating.
Finally, historical melee combat is as much a physical contest as it is about weapon play. Slams can be used to knock the opponent down to a prone position with all his weapons unready (page 371) and at a -3 to defense. Grappling can be used to immobilize an opponent for a friend.
Look at Knockback on page 278. Basically, for every multiple of ST-2 on damage before armor is subtracted, the opponent is knocked back a hex. More critically it forces them to make a DX, Acrobatics, or Judo roll or fall down.
Don't forget to apply the shock roll once anybody takes damage. For example 4 damage means the person is -4 to all rolls until the next round (page 419). Any damage over HP/2 will force the target to roll vs Health for stunning and knockdown (page 420).
The biggest issue the player will find when using these options isn't their complexity, but figuring out when they can be effectively used. For example, trying to slam a target with a lot more HP than you wouldn't be a good tactic.
Finally, remember the NPCs' morale. Rarely will an intelligent person fight to the death. Some will run away with a just a broken sword arm – and in a two person fight, that will likely cause the other person to run away as well.
There's no additional rule either in Basic set or in Martial Arts. As B391 suggests, you can defend against any attack from a known enemy at -2, except for situations when the defending character doesn't know from where (and when? This is not so clear) the attack is coming.
A character with the Peripheral Vision advantage can defend at -2 even if approached from behind. What such a person sees is just a shade somewhere in the extreme periphery of sight, leading to a rough idea that someone behind is attacking. It is similar when someone jumps behind me: if I can't turn, I can just do a "wild parry" (which means -2 to the roll).
There is another interresting problem: will the fighter with an enemy in his back (assuming he doesn't want to turn, perhaps due to another enemy in front of him) be able to defend in subsequent turns? The rules are not clear on this.
I would assume the answer is no: the defender knows that someone is behind him, but he can hardly expect when the strike is coming. Anyone who ever fought with a sword (including wooden/ plastic LARP swords) would agree that timing is crucial while parrying. So my quick and easy answer is that you can only defend against the first attack. The longer answer (how I solve it in my game) is as follows, but it is rather a house rule than an official rule:
The defender can try to dodge at -4 (-2 for not seeing the enemy and extra -2 for guessing when the attack is coming; Peripheral vision buys off the first penalty and Danger sense the second if the defender wins PER vs. higher of attackers Stealth and combat skill in the turn), moving either constantly or from time to time (all-out defence or defensive attack). Parrying or blocking is trickier. The defender can choose to swing his blade wildly behind his back, allowing him to defend at -2 - but this is for whole turn, which means this is possible only when the defender chooses all-out defence (Double Defense with one hand with a weapon/ shield committed to this enemy and useless against any other attack). Or he can defend at -4 (extra -2 for not knowing the timing). If attacker chooses "wait until there's no defence", no defence should be possible unless all-out defense was chosen by the defender (the wait maneuver would waste the attack in this situation).
Best Answer
There's GURPS, and then there's GURPS
I've played GURPS since it's first (medieval combat only) version came out in 1985. I've memorized most of GURPS 3e and have had lots of success running GURPS for new players. But even I find the 4e Basic Set a bit overwhelming.
Even though most of the core rules are very similar to the core rules of the 1986 1e Basic Set, because the 4e Basic Set really embraced the U for Universal in GURPS, and worked in most of the rules for supernatural and inhuman traits from the countless GURPS worldbooks that had come before, and also went further with the G for Generic in GURPS and added systems for modifying traits and working out what the point costs ought to be. The 4e Basic Set is over twice the length (~576 pages) of the 3e Basic Set (256 pages, 30 pages of which are basic Magic and Psionics systems rather than core rules), and most of the added content I never use, because I run games in low-tech worlds with pretty standard humans and fantasy races.
The 4e Basic Set is a great toolkit for designing almost anything, but it's got an awful lot of noise in it if you're a new player just looking to learn the rules so you can play a character in a game. Especially if it's just going to be a normal human character.
That said, I do have several suggestions for how to make even GURPS 4e accessible to new players, if you (the GM) have learned GURPS 4e already. Pick and choose any or all of these.
First, choose a game setting and type to run.
This will limit what the game will include to what is relevant to creating characters for that setting, and what will be used in the first sessions.
You might want to choose a relatively simple context to start with.
Something that greatly limits what there is to think about, but is still fun.
Warn players about the seriousness of combat and the importance of what characters do during combat.
Explain to them what combat gameplay and tactics are like, so they don't put a lot of time into creating a character and then do something foolish in combat that is likely to get the character killed or maimed right away. And let them know that it's possible serious injury or death may happen anyway.
To that end:
Run a fun short learning game...
... that may not even be part of a campaign you want to run, with characters made just for that game, that aren't expected to necessarily be used later, where serious injuries and deaths are expected to be likely during the learning session. A brawl where most characters involved have few or no unarmed fighting skills is a great simple starting point that can nonetheless be very fun, and is also a baseline thing to have experience with. A low-tech arena combat is also a great choice. If you're going to be doing gun-type combat in your campaign, you might start with a simple gunfight situation.
Have a bunch of characters pre-generated that players can use instead of building a character-
or that players can use as examples or starting points to modify to make their characters.
Prepare an introduction packet for the game you want to run.
It can be half a page, up to several pages. Explain the campaign setting briefly. Explain the intended style of play briefly. Then list what the choices are for new characters in the campaign. This will be a much much shorter list than what's in the Basic Set. You may also list various appropriate backgrounds (Templates) and what characters with that background have. Then character generation is mostly about picking things from your intro packet, not swimming through the 4e Basic Set or other books choosing from a universe of choices most of which won't fit your game anyway. Probably they can create a character with just the free GURPS Lite and your intro packet.
Be available to create characters with the players before play starts
Mentoring: a little goes a long way.
During play, I have found anyone can play GURPS ...
if you explain situations and options to new players in terms of the gameworld situation, in natural language, not game terms.
Then let them tell you what they want to do in game terms, and if they're interested and ready to learn game mechanics, you can tell them how you're converting what their intention is to game mechanics, in as much detail as they are ready for. This works well because one of the great things about GURPS is that the mechanics are based on trying to represent the game situation directly in ways that make sense. Every game mechanic represents something relatable and logical. So you can start by describing the situation, and work a new player up to understanding how the game mechanics model the details.
Resources
The free GURPS Lite rules contain the basic core rules of GURPS.
Some people even play entire GURPS campaigns using just the GURPS Lite rules. (As someone who loves the detail and especially the hex-based Advanced combat system, I'd never do that myself, but many have done that, and it's not a bad starting place, especially just as a first thing to read to understand what the core came is.) The 3e version of GURPS Lite is somewhat different and is also recommended for learners despite being out of date for 4e.
Check out some of the articles on Mook's Game Geekery blog
This features posts for new players, and more advice and resources on introducing GURPS to new players.