I'm aware that Valve/Steam is a competitor, but I still wonder if there are any plans to somehow support running GOG games on the Steam Deck (without installing a different OS). I'm aware of the workaround to install Windows on the Steam Deck but at least right now I'd prefer to use the officially supported OS.

No support:
https://techraptor.net/gaming/news/there-will-be-no-official-gog-steam-deck-support

What kind of support?

Do you mean being able to install and play the Linux versions of GOG's games, wich should run without much assle. It's a computer running Arch Linux, what more support do you need?

Or get official support in case any thing go wrong with some game, since they only support Ubuntu? Yhey won't.

I hope you're not wanting Galaxy for the Deck, cause I see exactly zero point in it.

They confirmed no official support. That being said, running GOG games through WINE isn't that hard.

If you get Lutris it can get most games to work with premade wine or proton configurations.