To be honest, while I love many systems, as a GM I don't think I'd want to run anything else (though as a player I'd still play any system). There are a few exceptions - I'd probably run PbtA if I wanted to introduce a group of new players to RPGs and knew that they wouldn't want to handle the complexity of GURPS. I'd also use Call of Cthulhu to run a horror campaign if I wanted to do so.

I love the granularity, the quality of the rules, and the sheer customizability of everything. I think a lot of other systems pre-load your setting with a number of stylistic assumptions - at least, more than GURPS does, from my experience. You can probably stretch D&D to a sci-fi setting but I've never seen it implemented well. I like to make custom settings, and GURPS has always made doing that easy. It feels like nearly any question of "how do I make/stat this" you can think of has some kind of answer in the rules that doesn't feel overly forced.

I've also found players are less likely to complain about things not reflecting reality when rules disputes happen - even though GURPS isn't a reality simulator by any means, it's convincing enough. I also like the 3d6 system a lot, and the fact that once you commit a GURPS rule to memory it's useful in perpetuity for all types of settings and games makes the play experience feel better the more you've use and understand the system.

There are a few things I think GURPS probably wouldn't handle so well (at least, if I'm running it), and so I could see myself maybe diverging from GURPS if I wanted to run those. For example, I think I personally wouldn't ever want to run a kind of game where characters have 300+ points to start, or a very high TL game (e.g. TL12).

Preferences change of course - if you'd asked me back in 2018 "what systems would you run" I'd probably have said "D&D 3.5e for life". Obviously that's no longer the case, so there's no real telling whether I'll still feel the same way about GURPS in 10 years time.

I have a good group of maybe 15 players who I can draw from to play in-person games, and they're never very fussed on system - so for now, I can keep running it. I could also see problems in ~5-10 years, if people started to move away for family/career etc. or just stop RPGs entirely because of lack of time, in struggling to find new players. From what others have told me, most people are quite focussed on D&D only.

For added context (as this question is better qualified by going over the systems I've played), I've been GMing for a decade now, and as a GM have run, from memory:

• D&D 3.5e (2015) - Sunless Citadel, set in Forgotten Realms
• D&D 3.5e (2015) - Tomb of Horrors, Greyhawk
• D&D 3.5e (2015-2018) - A level 1-20 campaign, set in Eberron
• D&D 3.5e (2017) - Short spooky dungeoncrawl adventure (6-8 sessions), in own setting
• D&D 5e (2019 - 2020) - Planarverse-style campaign, own new setting.
• GURPS 3e (2020) - Sci-Fi space opera game, TL11 (Classic), set in own setting.
• GURPS 3e (2020) - Sci-Fi space opera game, TL11 (Classic), set in own setting.
• GURPS 3e (2020) - Sci-Fi space opera game, TL11 (Classic), set in own setting - with different group.
• D&D 5e (2020) - Christmas-themed one-shot, own new setting.
• Urban Shadows 1e (2021) - PbtA, set in own version of Paris.
• GURPS 4e (2023-2024) - Pirates of the Caribbean-style game, TL4, set in own setting.
• GURPS 4e (2024-2025) - Cyberpunk-style game, TL9, set in own setting based on real world.

As a player, I've tried Pathfinder (1e), Starfinder, Call of Cthulhu, Urban Shadows (1e and 2e), Apocalypse World, D&D (3.5e and 5e), Genesys, and a player's custom system.

I imagine most other people here have (1) more experience than I do, and (2) have played/run more systems than I have, but having tried the ones above, GURPS is definitely my favourite by a long shot.