He didn't build it in a vacuum, though. GURPS borrowed mechanics and ideas from other game systems like the Hero System and Champions, and the whole project benefitted from all the different RPGs and setting books being released at the time. So exactly who came up with the idea for a "generic universal game system," and what resources and inspiration they used, is always going to be a matter of some debate.
GURPS was grounded firmly in The Fantasy Trip, Steve Jackson's own earlier game. GURPS added the 4th stat (HT), and makde skills operate like narrow attributes (where TFT, they typically removed difficulty dice). TFT was already a multi-pool point builder. One pool for attributes being raised from base, and another (equal to IQ) for spells and/or Talents.
SJ has admitted that the GURPS point system was cribbed from Champions, but it's not a foreign idea brought in, as much as a better implementation of an idea SJ had in 1977...
At one point, I'd have rated it a B+ or even A- - but that was fall 1986. My opinion of it has steadily grown downward. It's not that the game itself is bad, it is that the player culture has grown in a direction I seriously don't like engaging with.
Likewise, per Roleplayer Issue 1, 1st ed GURPS was setting point costs by hours needed to learn it to a specified level. (I'm in too much pain to go dig out my G1e box). IIRC it was 250 hours per point.
Later, it was moved towards (but not achieving moving to) balance rather than training. It's been incoherent ever since, and this enables a good bit of fan min-max.
I prefer TFT by far. That said, I don't find GURPS appealing for multiple reasons -
1) the costs are too complex
2) the skill list is WAY too long. (counting the full list in C2, over 200 discrete skills, not counting TL/ variations
3) the combat mechanics are too crunchy in the wrong places for me
4) The setting books usually bend the settings to fit GURPS, rather than GURPS to fit the settings. The exceptions tend to be earlier. The most egregious was the Ogre setting book... the prior games and short stories all had the infantry (and all the vehicles) lobbing nukes, but the GURPS: Ogre book removes infantry tac-nukes... of the setting books, this one SJG had every right to retcon,,, but it was a betrayal of 20 years established fiction for compliance to a 10 year old game's 5 year old vehicle ruleset.
5) The "normal" point range has climbed. A few books just go so insanely high that they're essentially incompatible with others. Most notably, G: Lensman.