DaReXX: So, will it at least arrive in 2021 still? Place your bets...
fronzelneekburm: No
The more interesting question would be that when/if it arroves on gog, will it be feature-complete (as in: working multiplayer, Q64 add on)?
To which my bet is also No. (see the post above mine)
Besides having to likely come up w/ DRM-FREE installers for the game and EVERY Add-On here on GOG - they'd have to probably pull the Bethesda.Net required log-in stuff - there's the problem of all of the content and getting it onto the GOG Store.
We've already had Quake 64 & Honey (which are in the add-on sections); and Horde Mode added in Steam versions, Bethesda.Net version, and Epic versions. That's already a few updates there.
Updating the GOG version would be a pain for them, probably due to how different probably the Epic and Steam versions would be. Those versions are probably simple, as they both allow Bethesda.Net and other DRM-server walls there; GOG really doesn't. They probably just make a build of the game for Steam and Epic and then toss the appropriate client-app store DRM junk on top - and are done w/ it over there; all done, all set.
GOG version - eh, not so much. Gamers would flip if Bethesda.Net log-in's are allowed on here, on a supposed DRM-FREE Store like GOG. They'd have to remove that Bethesda log-in crap for every time they do a GOG version.
I would guess, they (Bethesda) would at some point just have to stop updating and adding content to the game at some point, after a while and all - before it even comes to GOG. It just would make sense for Bethesda, Id Software, MachineGames, NightDive, etc - to just probably wait until they finally decide to stop updating Quake: Enhanced everywhere content-wise where DRM and client-server crap log-ins are allowed....then do a GOG Version without the Beth.Net junk and w/ all of the add-ons in one big package there, so they just bring all of the Quake Enhanced content in one big swoop over to GOG and just be done w/ it there.