Under the games tab i only see Dungeons 3 with Galaxy, but i want a stand alone installer. Does this game require gog galaxy.

Is this version compatible with the steam version to play coop online

Hello,
as for your first question, if you open your Library in a web browser and click on Dungeons, you can see on the "DOWNLOAD OFFLINE BACKUP GAME INSTALLERS". I think that is the version without Galaxy. If that does not help, you can try to download the game with GOG Downloader.

As for the second question, I do not know, to be honest, since I play this alone.