Not sure what you mean by with or without GOG, but their whole thing is that none of their games have DRM.
AFAIK, you end up with identical installs even if you use Galaxy to download and install your games, and the installs will continue to work even if you uninstall Galaxy. The actual game files are exactly the same.
I think the installers boil down to convenient self-decompressing archives for getting the game files onto your machine.
If you have the game files for a GOG game installed using any method, those can be moved around, copied, and run with no problem.
Heroic can actually do that too, now.