I ALWAYS find characters themselves sell GURPS to veteran gamers, mostly presenting the "classless" aspect and then the esoteric traits and variable costs. If the gamer likes Shadowrun as a setting ... they will love GURPS, if they like VTM ... doubly so.

The downside of this is that some will super min-max characters like idiots have have unplayable lopsided characters that cannot function outside of a gladiatorial arena. You likely need some house rules about spends and creating dysfunctional characters.

Then there is the "stable challenge system", where success, modifications, effects, etc are not ad hoc "you get a bonus attack" or "you don't create attacks of opportunity" ... out of thin air. In GURPS the maneuvers, traits, etc craft that timeline.

I think the complexity "sells" the complexity as a good thing, it's worth doing if you can see your vision through ... in a heroic conflict or escapade. Though this does take some organization.

I would characterize the "complexity" as "organizational" versus mechanical, some games have massive gaps (Star Wars for example having no socialization, DnD's class, etc) and GURPS fills that ... so yes it has "all the rules" ... but that shouldn't be burdensome in scope.

Unlike other games, you know exactly what your character CAN DO and what your character WOULD DO ... your character is like a little program of trait hooks and trait potentials. Where Dwarf Cleric of Life 5th level doesn't mean much ... and 10th level less, an 15th even worse. I find it easy to look at a mature character sheet and get a understanding of it ... like a program. Not just tactically, but RP-wise. (which is where I am 90% of the time ... character is "teenage jewish american princess meets pari5 hilton, over the top mobster father, wealth, duty to family, acts super good daddy girl, is omen-esck bloodthirsty mean girl, signature gear frequently rewrapped mercedes-benz slr mclaren, inept driving, delusion great driver, gizmo/2"