I don’t know what GOG galaxy is but I assume it’s a storefront like Steam.

Yep, basically that. No you don’t need it to launch games, I would skip it if this is your only game from GOG.

Sounds like if you run by opening Galaxy and hitting play, it won’t be the native version and will run through Rosetta 2 but if you run it from your applications folder it’s the native version. This is a bit odd because that makes it sound like by default what you have installed is BOTH versions which sounds like an awful waste of disk space

I don’t game on Mac these days but that seems very plausible.

If you do want another launcher option, Heroic works on Mac. I’m not sure if it gives any different options on what version to install. https://heroicgameslauncher.com/