Darvond: What games are causing it to open?
While I am not using Galaxy myself, my understanding is that all games that are available through Galaxy, even if you install them using the offline installers. This is the main, or pretty much the only, reason I don't have Galaxy installed, because I don't want it to get lauched if I am just playing some simple old GOG single-player game.
The game shortcut that the installation creates in your Start Menu and/or desktop includes the part which open Galaxy. So if you want to get rid of it, your choices are:
1. Uninstall Galaxy (then it won't be launched when you launch the game, duh!)
2. Edit the shortcut to remove the part of launching Galaxy (sorry I don't know how to do that).
3. Create a new shortcut that points directly to the game executable in the game installation directory.
About the third one, I am unsure if some DOSBox games and such give extra complications, as there you don't point directly to the game executable as far as I know, but you must probably point it to the DOSBox executable and give the right options for it which DOSBox config files to use and such. Maybe with such games the option #2 is simpler.