DG2 Rules in Use 

The Big One: DECIDE.

Declare defenses before knowing the to-hit result.

Option 2, all attacks: Use the tweak for any attacks – bullets, beams, arrows, knives, punches – in one of the two flavors below:

2b, last-second defense: If you don't choose to defend and the TH roll succeeds... wait, it's not over yet. Instead of an "immediate defense", you delayed for an extra split second to gauge the attack's exact path (i.e., to check TH) – and have confirmed that mace-on-head impact is indeed imminent. Time for a last-second defense!

You can make a last-second defense at a penalty: -1 for a melee attack, -2 for a thrown weapon, -4 for a missile like an arrow (knife, rock, bottle, Molotov Cocktail, etc.), -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.)

The Dodge penalty versus a laser beam would be infinite. If you didn't dodge the perceived path of fire before the trigger is pulled, there's no way you can even see, let alone dodge, the beam after it leaves the gun. (It's not a problem in a cinematic game, where "blaster" beams travel at a speed that makes arrows look fast.)

GURPS High Tech: Optional Wounding Rules (p.HT162), Body Hits, Limb Hits, Stopping the Bleeding, and "You shot me, Mister!" are all in effect.

Pulling Rank: See GURPS Action! 1, p.24-25.

Cellphones: The generic cellphone that comes with your standard of living has no special features. A Good-quality phone ($500), has two special functions (voice recognition, push-to-talk radio, GPS, digital camera, digital video camera, wireless connectivity), and provides a +1 (quality) bonus to skill. A Fine phone ($2000) has five of them, and provides a +2 (quality) bonus. Note that these gadgets count as improvised equipment (-5 quality) for anything but the most mundane uses. 

Weather and Terrain Penalties:

Rain:

Light Rain (-1 per 100 yards); (Medium) Rain (-2 per 100 yards); Heavy Rain (-3 per 100 yards). Applies to Hearing checks, Vision checks, and as a penalty to Survival checks.

Lightning: -1 to -4 to Vision and Hearing rolls for being outside near frequent lightning (multiple strikes per minute). Protected Vision and Protected Hearing work as usual.

Snow: Light Snow (-1 per 100 yards); Medium Snow (-2 per 100 yards); Heavy Snow (-3 per 100 yards). Applies to Vision checks.

Fog/Mist: -1 to -9 to Vision checks.

Wind: up to -4 to attack with projectile weapons, -1 to -8 in Hearing penalties (for an imminent hurricane or tornado).

Surfaces:

Gravel, loose sand, mud, snow: -1 to -4 to Stealth, same bonus to Tracking.

Slippery or unstable surfaces: Mud, slick pavement, etc.: -2 to attack, -1 to defend, -2 to -4 to Climbing, Jumping, Running, or misc. DX checks for movement (to grab the wet rungs of a fire escape ladder, for example), penalties (-1 to -3) to control rolls for mounts and vehicles.

Especially difficult terrain may necessitate slowing down to avoid risking falling down (DX check to avoid).

Reasoning: These rules are an attempt to differentiate combat areas. A fight in a windy thunderstorm vs. a fight in a dark auto repair shop with oil all over the floor will be very different...to avoid every combat being set on a infinite featureless plain (I think it's something that's been missing somewhat.).