I have the game on GOG but noticed that the Steam version has the 1.22 version (updated on January 22). It brings some performance and stability fixes. Is this patch already the one GOG currently has or has it not arrived here yet? As far as I can tell the version here on GOG is 1.0(B) (for rebirth 1), but I don't know if they have different versions for different platforms.

You're right in that GOG version is 1.0(B). I'm also not sure if this is a separate build from the Steam version, though I would assume so.

Your best bet is to write to GOG support and ask, preferably with a link to any changelogs posted on Steam, asking if the GOG version is on par. At least, that's what I've done in the past - sometimes the game gets updated, sometimes not... depends on the developer, really.

If I remember correctly, RB1 Version 1.0(A) was the one released on December 4th when the new DLCs were added, and the Version 1.0(B) was released around January 15th and never had any changelog or anything like that...
Maybe they are the same but have different version numbering between stores?