>Have you tried a source port instead?
Actually, I played DirectQ and it was nice. But I'd like to play vanilla game, that I bought on GOG.
It isn't always possible to play games 100% vanilla. For instance, 16-bit games when you have a 64-bit OS just won't run. So they either need emulating to allow vanilla play (like those pure DOS games needing DOSBox) or they need a source port to allow them to run on modern OSes and hardware.
When it comes to source ports, some are extremely faithful to the vanilla game, whilst others add lots of bells and whistles to modernize the game. For me, I've always found Quakespasm to be a very vanilla style port. Never tried Fitzquake so I can't comment on that, but as Syrion mentioned, Quakespasm and Fitzquake seem to be related.
Edit: Oh, and reading about Quake's history, GLQuake is a source port of sorts, but it also isn't a very good one as it doesn't support all of the game's graphical features (https://www.quaddicted.com/engines/software_vs_glquake
) and getting it to run on modern machines can be difficult, if not impossible. Even back when I used GLQuake on a previous PC a good number of years ago, I always had to manually crank up my monitor brightness as GLQuake's brightness setting did nothing at all, regardless of where you positioned the slider, and this was a known bug.