Core Mechanics of Gurps

Success Roll

Target number (An Attribute or Skill) + Modifiers (from -10 to +10 / negative is harder, positive is easier) vs 3d6 where rolling the target number or lower is a success. 3-4 is a critical success, and 17-18 is a critical failure.

Combat Roll

Attack success roll to see if you can possibly hit, followed by a Defense Success Roll to see if the target avoids or blocks the attack.

Damage

#d(+/-)# indicates the number of dice to roll with any penalties to the result.

Quick Contest

Success Roll vs Success Roll comparing the margin of success.

Self Control

Success Roll vs a fixed number. Used when outside factors compete for control of a character, such as an alcoholic passing a bar without going inside.

Reaction Roll

Using the NPC reaction table to determine NPC responses. 3d6 + Modifiers -> look on table