Steam needs to be installed to download and update that version of the game, but other than that is not required (if you don't care about the online lobby, friend invites or eventual achievements). You can exit out of Steam and start the game directly from the executable.
The GOG version has the (optional) Galaxy client for automatic updating, in addition to the full installer downloads and separate patches (not all patches are kept online, so if you miss more than a couple you'd need to use the Galaxy client to update or download the latest full installer).
There have not been any announcements about other platforms.
