They don't need to support the Steam Deck, they just need to support linux. Funnily enough GOG was one of the first platforms to try to bring the games to linux.
Which is why it's even weirder to me that they still don't make GoG Galaxy available on Linux. It stinks, even for Windows games on Wine/Proton, because you can't access cloud saves or online services without GoG Galaxy running.
I've managed to make it work with a hacky script to launch Galaxy under Wine, but it's fragile, crashes sometimes, and I really just want something better.