I played some GURPS in the 80s, probably 1e or 2e. I did not get a core book myself until 3e. Mostly generic fantasy, I remember playing a goblin. Then I played some in the 90s in a non-fantasy post apocalyptic game as a guy with a baseball bat.
I got a bunch of 3e GURPS sourcebooks and used a couple specifics in my AD&D games, in particular cyberpunk and Cthulhupunk drugs as potions in one of my Ravenloft games. GURPS's 3d6 system mechanics translates to a d20 one well enough even though it is a bell curve to a straight even distribution. Most of the mechanics I used were intuitive matchups.
I really liked a lot of the sourcebooks. When I was working on an NIH study on regulation of human enhancement technology I was tempted to get the GURPS biotech book for consideration of some possible future uses.
I also was a contributing author on GURPS Magic Items 3 with some items designed for Technomancer and Celtic Myth and generic fantasy.
Mostly I consider GURPS as similar to D&D usind a 3d6 instead of a d20 but you have 10 hp and no level advancement, just point buy everything and advancing by a few points at a time.
The skill system is very granular, my preference is for broader skill systems. How many skills you need to execute a concept in GURPS can vary widely, while the system rewards extreme specialization and working out the math for optimization.
I dislike the scale of one second combat rounds with very high impact damage and little damage absorption, driving everything to be the quickest attacks you can desperately get off as opposed to cinematic duel combat. I remember concluding that if I were to run it I would only ever use the simple combat options and not the full one with facing and such.
The completely open point buy allows huge variance in PC combat ability with no real floor. A 200 point scholar is really not a match for an even 50 point thug opponent who spends their points on combat stuff. I prefer characters to be balanced for combat.
I did like that it had systems for increasing skills in downtime if you focused on them, more than just spend your xp as character points to improve them.
Mostly the skills had very little description about what a successful or failed roll meant, or how to use them, unless you got into specific sourcebook stuff like how to hack computers in GURPS Cyberpunk or what the modifiers are to alter someone's genome in GURPS biotech.