I’m just setting up my Raspberry Pi 4 and would like to install a GOG client there. I need to pick an OS at the moment and need more info to make a correct choice.

There are available:

Raspbian
LibreELEC
OSMC
Recalbox
Lakka
RISC OS
Screenly OSE
Windows 10 IoT Core
TLXOS

Also, Ubuntu Mate is coming soon.

Which ones are going to be supported by GOG Galaxy?

Thanks!

The Galaxy 2 download page said that: "GOG GALAXY 2.0 requires Windows 8 or newer. Available also on Mac OS X."
So we can assume that those would be the supported OS.

AFAIK linux clients is not planned yet.

Leaving aside, that there is currently no Linux version planned (at least not in the near future), you will get a plattform problem:
even if you find a way to install the galaxy client, which is targeted only for x86 Plattforms, on an ARM Plattform. Every game the Galaxy Client can install* is also targeted either for the x86 or x64 plattforms, so no dice.

* as integratrions are mostly user made, it theoretically possible that one could create an integration for it, but still then, someone would need to create packages for games were arm build exists.

I would say its better to use the package manager the distribution offers to install the games that are available

oooh.. so the question I just post is out of date.
So Galaxy 2 do not work with windows7 ?

It works fine on my Windows 7 notebook. It's just not supported by gog, so if any update will break the functionality, they probably won't fix that.

isn't the same than "do not work on W7"? if i use it and then it is broken and never fixed, i will lose my games. ? Better not start to use it no ?

If it doesn't support Windows 7 then it's a no-goG for me.