I just bought a game from Greenmangaming and got a Steam key. However, when I went to activate it on Steam it said I activated the UK version of the game. I live in the US and want to know if there will be any problems playing it and which version of the game will be installed.
The answer to this is in the Steam Region Restrictions FAQ
However, said answer is anything but clear. Basically, for CD Keys (which these would count as), it says
Region restrictions on CD Keys vary by each individual publisher. There is no standard or universal restriction placed on retail games.
Having said that, I haven't had any problems activating keys from GmG in the past. Granted, this was only three titles: Murdered: Soul Suspect, Alien: Isolation, and the Alien Isolation Season Pass.
You'll know if the key was rejected, as it'd tell you so when you try to activate it.