This is (probably) all 3rd-edition specific.
Damage and Danger
All-Out-Attack damage (AOA)
GURPS allows characters to add +2 points of damage with an All-Out Attack. But a straight +2 means +2 whether it's a slap from a ST 6 weakling or a battleaxe smash by a ST 17 bruiser.
For a more realistic effect, add +33% damage, rounding up, instead of +2 points. This will please the high-ST players in your game, and provides appropriate results for characters of any size.
Effects of Damage
Slowdown from injuries
Halve Move and Dodge when HP are reduced to less than x1/3.
You fall down on damage of over HP x1/2 and a failed HT roll.
This mirrors GURPS' knockdown rule, and is separate from falling down after knockback from a collision.
An arm or leg is crippled on damage in one blow of over HP x1/2, a hand or foot on damage over HP x1/3. Excess damage is lost.
A single cutting blow inflicting twice the damage needed to cripple a limb (i.e., over HP damage for an arm or leg, over HP x2/3 damage for a hand or foot) can amputate it on a failed HT roll. Amputation is automatic if damage is twice that amount, on a sliding proportional scale from -0 to -HT.
[Check it out].
Actual damage taken, though, is equal to the amount needed to cripple the limb; excess damage is lost.
Crippling damage for a neck is the same as a leg or arm.
A crippling blow "cripples" the neck and paralyzes the body (attempt recovery roll normally). An amputating blow decapitates, or fatally breaks the neck if crushing.
Roll vs HT each turn while at HP 0 or less to avoid unconsciousness.
Roll vs HT at -HP and every -HP x1/2 thereafter. Automatic death is at -HP x5.
Aim is like Wait -- you can stop aiming (and sacrifice your aim) if something happens with your target. Example: You declare Aim at the Bad Guy across the room. He spins and starts running at you and has enough move to Slam you on his turn. You can sacrifice your Aim and fire. This only works for finger-twitch missile weapons like crossbows, bows, and guns -- not an axe or shuriken. If you want to wait until Bad Guy gets closer, you can wait til he's two yards away -- to fire when he's even closer but before he slams you requires a Will Check (Combat Reflexes gives +1, with a penalty equal to the bonus you would receive for the reduced range).
Attempt a Pin
If you declare the defense and the blow strikes true, roll the defense normally.
If you declare the defense and the blow misses, then you're okay. You dodged or threw up a block against a blow that would have missed anyway, but hey, that's normal in combat. Your head's in one piece; quit griping.
If you declare the defense and the blow was a feint, you were faked out; you reacted defensively to a blow that never came. (This is a very likely situation in combat, but it'll only happen in GURPS if you're using this rule! You can further tinker with the system as you like; if you win the feint Contest, the GM can rule that you saw the ruse and didn't waste the defense.)
If you don't declare the defense and the blow misses, you lucked out – and probably looked pretty impressive (or nuts), standing pat as bullets chew up the wall around you.
If you didn't declare the defense and the blow hits – wait, it's not all over yet. You played wait-and-see – and are now thinking "oh, @#%!!" as that mace screams toward your head after all. Time for a panic defense!
Last second: You can throw up a last-second defense at a penalty. Call it -1 for a melee attack, -2 for a thrown weapon, -4 for a missile like an arrow, -8 for a bullet, or -16 for a hypervelocity round. That's pretty well impossible for bullets and faster – because it represents actually dodging or blocking the bullet itself, not jumping out of the muzzle's path. (Some cinematic heroes still manage; all you need is an obscenely high defense score.)
Seperate AD and PD
Compute AD roll as 2 + the chosen Active Defense score, and PD roll as 2 + the total of all passive defenses. A character with PD 4 from armor and magical protection, and Parry 6 from Broadsword-12, has a PD roll of 6 and an AD roll of 8. When a combatant actively defends against an attack, roll against AD first; only if that roll fails do you roll against PD.
Advanced rules: An AD roll that fails by only one almost avoided the blow: add 2 to the following PD roll and halve damage if the defender is still hit. A PD roll that fails by only one almost deflected the blow: halve damage. Quickness helps you avoid the worst of a blow, even if you do get hit: "It's only a cut – but if I had ducked any slower, I'd be a goner".
On the attacker's side, an attack just making its TH roll isn't a very good one: the defender gets +2 to his AD roll (or PD roll if he doesn't defend). These options help lower damage even if the character doesn't fully avoid a blow.
Dodging Huge Weapons
There's no Dodge penalty if the weapon's width is one-third your size or smaller. There's a -2 Dodge penalty if the weapon is half your size (your Size minus 2), a -4 penalty if it's two-thirds your size (your Size minus 1), a -6 penalty if it's your size, a -8 penalty if it's one-and-a-half times your size (your Size plus 1), a -10 penalty if it's twice your size (your Size plus 2), and so on.
Retreats: The Retreat bonus lets you subtract the Retreat distance from the weapon's effective size for Dodge purposes, as well as adding its usual +3 to Dodge.
Close calls: For more detail, a blow is dead-center when it would have hit without any aid from the weapon's large size. But if the TH roll isn't quite as stellar, the weapon is a bit off-center and easier to Dodge. For each point by which the TH roll missed dead-center success, treat the weapon's width as one Size smaller for Dodge purposes.
Retreat distance: You can rule that a character's normal Retreat distance is his Step distance, not just a single hex. This wouldn't add to the +3 Retreat bonus; a big Step gets the combatant away from the attack further, not faster. But it can help a lot when Retreating out from under dinosaur stomps. If you can make a 2-yard Retreat, dodging a 6 foot-wide weapon is no harder than dodging any other attack, and the +3 Retreat bonus counts too. Your roll: full Dodge, +3.
Dive for cover: One final topic: couldn't a character use a broad jump or a dive to get out of an attack's way? As above, this "Diving Retreat" would give no more bonus than the regular +3 AD Retreat bonus. But it might surprise an enemy, take you behind cover, or take you out of range of another attack. Most importantly, it could move you farther out from under danger than your regular Retreat does.
The character should be ready for it (Wait maneuver, or All-Out-Defense). Make a balance or Acrobatics roll at -4 to land on your feet.
Parrying Huge Weapons
Compare the "oomph" of the attacking blow and the defending parry or block. "Oomph" here is a very scientific term for force, and is equal to Load ST + weapon or shield mass.
Add 50% to Load ST for these purposes for either fighter if using two hands on a legitimate two-handed weapon. Make that a 100% bonus for a defender using two hands with a wide grip (polearm, staff, etc.). In either situation, add another 10% Load ST for any hands over two on a weapon.
If you defend against a relatively high oomph you'll get a Parry/Block penalty. If that penalty causes your defense to fail, your weapon or shield is overwhelmed by the powerful attack.
Assess a penalty to parry equal to ( (attack/defense) -1). So 15/10 is no penalty, 22/10 is a -1, 36/10 is -2...
Rules for target size and melee attacks
Apply the difference between attacker and target Size as a TH modifier with melee attacks.
The max bonus for a target larger than yourself is +2 TH (or +3 with a weapon your length or longer).
Rules for weapon size and melee attacks
If your weapon is particularly wide compared to the target's size, add its width to the target's size; use the TH modifier for this combined size.
Character Points, Training
Weapon Master advantage acquisition
If you already have a weapon skill of 16, you can begin putting character points in the 20-point Weapon Master advantage (CI-32).