GURPS Rules in use:

GURPS Basic Set: Characters

New skill specialties: 

Merchant has two new specialties: Illegal Goods and Stolen Goods.

Tactics is split up into the following specialties:

Land, Air, Sea, Space, Police, and Criminal.

GURPS Basic Set: Campaigns

p.B420, Bleeding: This optional rule is in use. Hard to Kill and Hard to Subdue, do not apply to the Health roll, Fit or Very Fit (as well as any special modifiers for coagulants or alien racial perks, etc.) do apply. 

GURPS High Tech

p.HT198, Sheaths: These rules are in use.

GURPS Low Tech

Available Firearms
Long Guns:

Arquebus, Caliver, Musket,
Winged Tiger Gun
Carbine, Wall Gun
Fusil de Chasse, Fusil Ordinaire

Blunderbusses and Shotguns:
Blunderbusses and Fowling Pieces (single or double barreled) are both available.
   
Pistols:
Military Pistol
Dragoon Pistol, Heavy and Light.
Petronel

GURPS Magic

p. M67, Deflect: Deflect now ONLY applies to Dodge.

Alternate Enchantment Prerequisites:

These replace the normal requirements for Enchant.

Magery 3, Thaumatology at IQ-3, 1 point in a spell with at least 5 prerequisites.
Magery 2, Thaumatology at IQ-2, 1 point in two spells, each with at least 5 prerequisites.
Magery 1, Thaumatology at IQ-1, 1 point in three spells, each with at least 5 prerequisites.
Magery 0, Thaumatology at IQ+0, 1 point in four spells, each with at least 5 prerequisites.

Quick Alchemy Rules

These rules allow for useful potions and elixirs to be made in the field, in days, not weeks or months.

Alchemy note:

Note p.B438, Poisons, Delay--each +1 to SM doubles the delay of a poison, each -1 to SM halves it.

GURPS Martial Arts

These optional rules are in use: p.MA68, Targeted Attacks; p.MA80, Combinations; p.MA100 and p.MA 109, the uses of Evaluate under Countering Feint and Deceptive Attacks and Evaluate and Non-Combat Skills; p.MA100, the bit under Feints about resisting with your best skill, not just the one you're using, and Beats (same page) for unsubtle attempts at penalizing defenses.

See p.103 Fast-Draw from Odd Positions and p.MA104 Quick-Readying Nearby Weapons for grabbing weapons during surprise attacks.

p.MA111 Reversed Grip allows for a tad more impaling damage and the use of unarmed parries.

For alien monsters, animals, and toothy sentients p.MA115, Teeth will be a useful reference.

p.MA118 Shoving People Around; p.MA119, Pain in Close Combat and p.MA118, Grab and Smash! allow for hostage-takers (literally) strong-arming someone.

p.MA137, both New Hit Locations and Notes for Existing Hit Locations are in use.

GURPS Thaumatology

Resonant Designs and Materials, Thaumatology p.110

Optionally, the GM can reduce enchantment costs by 2% per stylistic feature that makes the item blatantly appropriate
for its enchantment, so that anyone who looks at it and knows that it’s magical can guess some of what it does. The
viewer should at most require Cultural Familiarity with the maker’s culture, and perhaps have to make an IQ+4 roll. If
the symbolism and clues require close inspection, or a roll against a skill such as Occultism or Thaumatology, then the
reduction is only 1%. Such details also tend to be stylish, flashy, or rich – thieves should certainly be strongly tempted
on sight!

Quirky Enchantment
Failures or critical failures give a magic item quirks or odd features, they do not waste time or materials. Critical successes add slight benefits or cancel out critical failure effects.

House Rules

Cocked dice: Dice that roll off the table or do not land flat are void, reroll.

Tattoo-based Energy Reserves are limited to a number of FP equal to the subject's HP.

GURPS Terrain & Weather Modifiers:
Surfaces:
Gravel, mud, snow: -1 to -4 to Stealth, same number applies as a bonus to Tracking rolls.  
Sand (dry or wet) gives the bonus to Tracking but not the penalty to Stealth.

Rain:
May create bad visibility (-2 to attack, -2 to Vision and Hearing, may wash away/muddle scents, -2 to Smell rolls).
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.
Lightning: -1 to -4 to Vision and Hearing rolls for being outside near frequent lightning (multiple times per minute).
Fog/Mist: -1 to -9 to Vision checks.
Wind: up to -4 to attack with projectile weapons, up to -8 in Hearing penalties (for an imminent hurricane or tornado).