So the Linux version came out on Steam. So will GOG get the Linux version?

Probably, but for whatever reason the people at GOG don't work on days of the week that start with "S", so you will need to wait until tomorrow at the earliest.

The Linux version is now here :)

Ah, too bad I already did my first playthrough using wine.

The saves may be compatible, though. Tried copying drive_c/users/<username>/Local\ Settings/Application\ Data/UNDERTALE from under the WINEPREFIX as ~/.config/UNDERTALE_linux, and it is at least starting from the end as it should.

Linux version is now up on GOG!