When I used to run several
GURPS games at each of three conventions per year, I did two primary promotions:
1) I ran some games with titles that would sound familiar to just about every one. For example, "GURPS Monty Python" could get me Pythonites who'd never heard of
GURPS.
2) I'd says, "If you can roll three six-sided dice, you can play
GURPS." Players would tell me what they wanted their PC to do, and I'd tell them what to roll on. Simple as that.
Note that when I started GMing
GURPS, I had never played it and none of my players had ever played or GMed it.
EDIT: For conventions, I almost always built the PCs in advance. The primary exception was for
GURPS Davenford, which was a fantasy campaign set in Megalos, Yrth, with several sessions run at three annual conventions (OrcCon, Gamex, Gateway, all Strategicon). For that, players designed and played their regular character(s).