GOG's problem with Linux is that Valve® pretty much owns the place given its Feral-ports of AAA titles, Steam client, involvement into drivers, SDL and other libs development and now the WINE^W Proton™ move, while GOG has basically nothing (its roots are in the old games which are invariably either DOS™ or Windows™ based).

Compared to Valve GOG is a small pest and GOG's top brass probably knows it, thus preferring to wage its own war (like "GOG Connect" and "FCK DRM" schemes) trying to lure as much of the Windows users from Steam instead of battling a lost battle (GOG's involvement into Linux can be summed up as "Too Little, Too Late", given that GOG was the last to provide any sort of Linux support).

But in case Microsoft® succeeds with its Store all things are suggesting GOG will be screwed, plain and simple.

Right now there is almost no incentive for a Linux user to use GOG.

Steam only owns GNU/Linux because GOG is not releasing Galaxy on Linux. Many of the newest games already offer native Linux versions of their games. GOG is letting the ball fall off their hands by not releasing a full featured Galaxy version.

They could even use snapcraft so it's distribution agnostic

https://www.youtube.com/watch?v=DLxqdf89hRo

Then it could simply be installed with the snap command that's already being distributed by default in distributions like Ubuntu.

I'd say it's there is the same incentive as for windows users: because it's DRM free. This may not be concern for the majority of gamers, but apparently it is for enough to keep GoG alive.
Not sure if that's the sole reason given that a large portion of GoG's customers doesn't use galaxy because they don't like third party clients managing their games to begin with.

GOG is cheaper in my region and I've been buying most of my games here a while since.

GOG does an excellent work at restoring old games by doing it correctly instead of leaving shovelware there as Steam does.

I only use Steam for exclusives and bundles from Humble Store and Fanatical.

Well, not according to business documents.
See their Capital Group presentation - H1 2018 at page 13. Net profit of -3% and -1% for Q1 and Q2 of 2018 respectively.
On top of it, according to Consolidated Financial Statement for H1 2018 (page 43) roughly 2/3 (64%) of sales revenues come from CDProjekt's own products. 3rd-party developers are not that interested in CDProjekt's distribution channels. :(
And when it comes down to Linux, Steam™ has a metric ton of exclusives right now. And some high-profile to boot. :(

Really, last I saw they were either chucking out the same rubbish steam was, for more money, or broken versions. Maybe price is better in your region.

It is cheaper in my region.
Some games are around half the Steam's price.
Games like Factorio used to be cheaper when I bought them and they can get their automatic updates from the developer's website. So I'd only miss the achievements (some achievements are like milestones so you feel progress).
Now I want to buy CrossCode and I could pay double the price on Steam and have the same services on any OS.
I only wish GOG did the same. It'd be a no brainer to get all of my games on GOG.