Pretty much how you "level up" in GURPS is by character point awards you spend on your character.
Or advantages gained in play.
But our Gamma Terra game has another twist - a random "level up" table.
I don't have the whole sheet, but it was percentile based and had a wide variety of boosts on it - Charisma +1, IQ +1, ST +1, Hard to Kill, etc.
There was even a 1-in-100 chance of "pick one" and the same chance of "roll twice, pick one."
Me, I rolled DX +1.
Since we started I've picked up 25 points, plus 1-2 levels of Night Vision (a mutation), Distinctive Feature (cool white eyes), and now DX +1 on top of that. I spent 10 of those points getting +5 HP (it was on our list of pre-approved advantages), leaving me 15. I expect 5 more points next session.
Personally I'm debating +2 to ST as soon as I have some downtime to justify bulking up a bit. Or I could improve some skills, or up DX again or up my IQ. Or split it to +1 ST and +1 HT.
I'm not sure, although I'm leaning to +2 ST or +1 to ST and +1 to HT. I like saving up and getting a big whack of power all at once, instead of spending ASAP and getting the most immediate boost I can. 5 points a session means a 15-point advantage every third session, if you don't spread it all over the place. Although Wrestling at DX+2 would be nice, and it's only 4 points, so that might happen sooner rather than too much later.
All in all, I'm about 41-42 points up from the campaign start. Still not enough to survive Gamma World without some smart play and luck, but it's really nice.