Tannath: What if one has the Steam client installed on their PC? Couldn't that cause some kind of interference if the GOG build kept the Steam stuff in there?
I have Steam installed on this PC, which is how I downloaded the game & where I've been testing it (I've also tested on another system that's never had Steam installed on it and it works fine there). I can't see any way it would interfere with Steam unless the user went out of their way to make it do so.
From what I can see it does not interact with Steam unless it's either
A- Run directly through Steam, or
B- Has a steam_appid.txt present in the same directory as the executable with the game's AppID in it and is started while Steam is running
Running it without the Steam client running or present on the system results in this:
[S_API FAIL] SteamAPI_Init() failed; unable to locate a running instance of Steam, or a local steamclient.so.
While running it with the Steam client running in the background results in this:
[S_API FAIL] SteamAPI_Init() failed; no appID found.
Either launch the game from Steam, or put the file steam_appid.txt containing the correct appID in your game folder.
And it fails to connect with Steam - Steam doesn't recognise that it's running, can't get achievements etc. In both cases it runs fine in a Steam-free mode.
QUBE's savegame data is stored in the same directory as its executable, so I can't see how cloud saves could possibly be affected unless you installed it right into your SteamApps/common directory, overwriting the Steam-installed version and savegame (and in that case, *why* would anyone do that?).