Why don't you provide the Linux version of the game but just the Windows version?
I know ID Software released the native version of the game in the past so you should have it too.
I have the file version linuxq3apoint-1.32b-3.x86.run but I'm not sure if it is the latest possible version.
Can you provide the latest version for Linux or is there some license problem with it?

Use this for linux: https://ioquake3.org/get-it/test-builds/
And this to find games, CRom servers are always populated: https://www.gametracker.com/search/q3/

It's up to the publisher to publish their game. GOG can't publish the linux version if id/zenimax doesnt want to publish it.

But fortunately we have ioquake3 and many other free quake-like games.