[Gef]

In my experience, different is good. For awhile in 3e days, I'd do demos with 2 adventures prepped, standard fantasy fare and something you can only do with GURPS. It also helped that I could pretend to be a carnival barker to drum up a bit of enthusiasm. "Come try GURPS! It'll be a lot of fun! Takes about 4 hours. You'll be playing a gladiator in ancient Rome with mutant super powers!" Talk with exclamation points. Yes, I used pregens.

The other way I've gotten gamers to try GURPS is to join a pickup game of someting else, contribute, have fun, be a guy people would like to game with. And oh, thinking like a GURPS player, have my character do things that would be easy in GURPS, maybe even explain how you'd do it in GURPS as you ask the GM what he wants you to roll. Most games don't have the flexibility; poor GMs will say you can't do it, good GMs will wing it, but either way, you've piqued some interest.* So in a few weeks when you run your demo, they want to see it in action. Beware: This tends to lead to people wanting you to continue your goofy one-shot demo as a full campaign with all the world-building that entails. For that reason, I'd suggest real-world, modern or historical, although it can have fantastic elements.

By the way, that gladiator game was very simple: One of the gladiators from the school of Hardius Knocksae has been kidnapped - whichever pregen none of the players picked. The lead suspect is the grudge rivals, the school of Dirtius Tricksae, but a few Carousing rolls later, and they're on the trail of that new school, Equus Darkus. They're also drunk, so they get mugged by some normal thugs on the way back to barracks. Super powers with DX penalties, fun intro to combat. Break into Darkus compound to rescue their bud, then into the arena, with their team back up to full strength, for some payback. Four hours easy. As I recall, some of pregens were as follow:

Tank - a 12' giant, lots of ST and HP, skin as tough as leather
Blaster - 3d lightning attack, fly by "electrostatic repulsion" max altitude 30'
Sneak - basically a tiger in human form, keen senses, super jump
Super-Normal - no powers but high skill
Speedster - looked normal and was a senator's daughter in it for the thrill
Morph - I only gave him 5 shapes, to keep it simple for a short game

Every time I ran this, a player told me GURPS was obviously unblanced, because one character was so much more powerful than the others. I was told this, on separate occasions, of the giant, the sneak, the blaster, and the morph.

Good luck,

GEF

*EDIT: Sample scenario, your guy lurks in the mouth of a dark alley until his mark passes, then steps out to grab him in a choke hold from behind. Plausible in most any genre, and in fact plausible for a low-skilled or unskilled hero because of the circumstances. In GURPS it's straight-forward: Stealth-vs-Per, bonus you because you're not moving and you're in a shadow and maybe never in sight if the mark doesn't glance up the alley, all-out attack to cover the distance and get a bonus to hit (not so risky since the mark can't hit back if A) you surprise him or B) you succeed in the grapple), mark defends at penalty because you're behind him. Again, with the bonuses, circumstances favor the attacker if neither party is unusually competent at rough stuff. Not every game handles this well. The fact that GURPS does so is alone insufficient reason to try it, but the fact that you can explain it simply would be a good reason to try GURPS with you as the GM.

PS: One note about pregens - don't make them too competent, even if you eventually hope to run a high-powered campaign with the same players. I'd even say, don't use cinematic combat rules. In other words, don't put yourself in position to explain, "With skill 24, you can target the eye at 15, or take a Rapid Strike at half penalty because of Weapon Master, for two extra attacks, and target the Arm at 16 for the first attack, the Vitals at 15 for the second, and if that doesn't finish 'em, a riskier to shot to the neck at 13 for the last attack. Note that the defender will be at a penalty to defend against your final attack." By the way, I've just started a campaign for new players, psionic conspiracy setting, 175 points but with 75 in psi powers, that makes them 100-pointers for mundane stuff like weapon skills. I tell them that skill 14 or 15 is pretty good, and I make sure it's true when I build NPCs, and they have no reason to whine that they can't snipe with pistols.