GURPS has a reputation for being an insanely complicated system… not without merit as there are LOTS and LOTS of optional rules that come together to make GURPS a highly detailed and very simulationist game. However, the way I tend to run GURPS games, using almost none of the extra “stuff” makes it into a pretty rules light system.
GURPS only uses d6.
first you have 4 attributes:
Strength (ST), Dexterity (DX), Intelligence (IQ), and Health (HT) which are pretty much exactly what you expect them to be. One thing that is a little different from what you might expect is that ST actually controls how many Hit Points you have, while HT controls your Fatigue Points.
Character creation and advancement is a highly detailed points buy system, it will mostly be behind the scenes as I will create the characters, and will go over advancement when everyone has enough points for it… basically tell me what you want to get better at and I’ll tell you how to figure out the cost.
Skills are many and detailed, and are based off of a controlling attribute (almost always IQ for mental/knowledge skills and DX for physical skills) Your character sheet will have your level pre-calculated for all your skills so there will rarely be any math you need to worry about.
To use a skill (or do pretty much anything) in GURPS, you roll 3d6 and attempt to get below your skill score. Modifiers are added or subtracted from your skill score before rolling. a 3 is always a critical hit, a 17 is always a miss and an 18 is always a critical failure. the critical hit range expands with higher effective skill level (skill+mods) and critical failure range expands with lower effective skill level.
Skills above 16 are still useful in spite of the cap of 17 and 18 always being auto miss because the extra points will help absorb negative penalties to the skill check that would otherwise lower it quite a bit (ie skill 22 with a -4 penalty still has to roll a 16 or less, while a 16 with a -4 penalty drops to 12 or less)
Combat is significantly different from D&D as it attempts to be, even in it’s basic form, significantly more realistic. Initiative is permanently set by the speed of your character, and does not change from combat to combat. Faster characters are always faster, and waiting/holding does not change your place in the queue. ambushes and things of that nature may cause a character to skip actions, but this is really the only way to “get the drop” on a faster character.
Combat turns (on personal scale) are all just ONE second long. This means that that much less happens each turn. moving and attacking for example is possible, but there are significant penalties to the attacks due to the limited time frame. depending on what weapons you are using, you may not be able to attack every round, and some rounds you may have to take a “prep” action if you want to be most effective (such as spending a round aiming a rifle, or taking a turn of melee combat to evaluate your opponents fighting style) remember that 6 gurps turns == 1 D&D turn and it will help with visualizing what all is going on.
hitting with a weapon is a 3 step process.
First roll to see if you hit, normal skill roll.
Second, the target may have a chance to take a defensive action, his available choices depend upon what he did during his last turn, what kind of attack is being made, and if he has already used a defense this turn or not. There are 3 types of defense: Dodge, Parry, and Block. For the most part in a sci fi game, Dodge will be the most commonly used defense due to the preponderance of ranged weapons. Parry and Block are really only useful for melee combat. the defender rolls against his defense score, and if he makes it, the attack misses. CRITICAL HITS CANNOT BE DEFENDED!
finally, damage is rolled, and armor’s Damage Resistance is subtracted from it. In GURPS all armor works by subtracting from the damage dealt. Certain types of attacks (like armor piercing bullets) may significantly reduce the DR of the armor, but only rarely will it be completely negated
whatever damage is left is applied to the target. The average human has 10 HP. Instant death does not occur until -5x HP, though there is a sharply increasing chance of death the further you go below 0, and all activities are significantly impaired
also if you take any damage during a turn, you have a shock penalty to all actions the next round equal to the damage (max of -4) due to the pain
and… that’s pretty much it… there are specific maneuvers you can take each turn, but they are all pretty straightforward – move (full move), attack, move and attack (full move + one attack at -4), all out defense (no attacks for the opportunity to use 2 different defenses or a +2 bonus to one), all out attack (multiple attacks or one more powerful attack but no defenses), aim (increasing bonus to next ranged attack), evaluate (increasing bonus to next melee attack), concentrate (keep spell/psionics going, making perception checks, leadership rolls, etc), ready (draw, reload, or regain grip on a weapon), do nothing