Therefore worst place.

Does anyone know why the Linux version does not exist on GOG?

Yes. I haven't run into that before.

Well, I don't use Linux, but I'm pretty sure if you search the forum, you'll find users commenting on other games missing their Linux versions.
I know, I've seen plenty of those.
Yet no explanation as to the "why" (other than "because Linux" and "because GOG").

Edit: typo

My conspiracy brain says they see open source software on around the same level as state owned software; being unable to trust software designed by the people for reasons which could range from loathing or dreading a design by a distant committee who doesn't deign to care what you think, that code being open makes it vulnerable to bad actors (a silly cold war thought), or that it'd prove to be too much of a moving target compared to using the same codebase for 80 years. (Build once, never move. Big iron curtain.)

My more rational mind says they backed the wrong horse in supporting Ubuntu, which itself can prove to be a nightmare to support since they were actually supporting Mint, which was a LTS of an LTS of an LTS.

The Steam version is supporting Ubuntu. So I don't see how any of that would be an issue.

Unless I'm mistaken, GG.deals doesn't sell anything themselves. They are a competitor to IsThereAnyDeal, and like that site, probably get small referral fees from purchases made on various stores after someone followed a link from there. One difference is that GG also lists stuff from grey-market shops like Kinguin (something ITAD doesn't seem to do -- which is a point in their favor), but you can at least filter those out by selecting "Official Stores" under "Store Type".

It's not just the Linux version -- the macOS build is missing, too.
Doing a quick Google, I see that the dev/pub contracted with Feral to make the Linux and Mac versions. Both Feral and Aspyr (another developer that specializes in Mac and Linux ports of Windows games) seem to strongly dislike the idea of DRM-free releases of "their" games. Since they generally seem to retain partial ownership over their ports, they have a say in where those ports get released (or, rather, where they don't).
Notice that nothing from Firaxis or 2K that's sold here has either a Mac or Linux version listed.

Unless I'm mistaken, Steam's OS is Arch based, which means they're supporting Arch, with Ubuntu being incidental in support.

At the time of XCOM 2 it was still Debian based, only around SteamDeck release they switched to Arch. But this has nothing to do with neither SteamOS nor Ubuntu as ports made by Feral are suppose to be distro agnostic and so is Steam itself (you don't need SteamOS to have official Steam on Linux).

Yup.
OK, thanks.