Steam version is exactly the same as GOG. GOG even comes with steam_api.dll, so go figure.
Not sure if the GOG galaxy integration will make the executable distinct at some point, or if it'll simply contain hooks for both Steam and GOG Galaxy (the easiest and most logical way of doing it).
Copying the installed Steam folder is easy. Everything is self contained in one directory, so it's not anymore a hassle than with GOG. Of course, if someone is unfamiliar with navigating the windows file system, then sure - that would be a problem. A five year old can do this though, so...
Steam is only used as a download platform. You do not need Steam running or even installed to play the game, once you have downloaded it.
