If I buy a game from GamersGate is it redeemable on Steam?

If it says "Steamworks" DRM, then yes.

What game are you trying to buy on GamersGate?

I don't know if it's the same on the dot.com site as the dot.co.uk site (I get auto re-directed to uk when trying to see) but it should say on the store page of the game.

At the top of the game summary section on the right-hand side does it have sections for "Platform, Categories, File Size, Publisher, DRM"? Look there.

It's different from game to game. You need to check the DRM details in the game description to know.

http://i.imgur.com/eXzIP1q.jpg