Although I haven't finished playing it yet, Metro Exodus is a really good game. It's an open world post-apocalyptic FPS with an intriguing storyline.

The first two games (Metro 2033, Metro Last Light) are also pretty good but aren't open world. Metro 2033 also has somewhat of a horror aspect to it. The world building is fantastic!

The entire Metro trilogy is quite good, but specifically Metro Exodus is probably the one most similar to Fallout. And AFAIK it's also available on GOG. (Last Light and Exodus also have native Linux versions)

I'd highly recommend the S.T.A.L.K.E.R series. The original three games are getting older at this point, but there are loads of mods available to improve/change/modify gameplay, visuals and darn near everything else.

@thekittencultist Stalker was great. More of a shooter than fallout, but had a great atmosphere.