If I buy Sid Meier's Civilization V on GamersGate I will get a Steam key?

The page I got says DRM: Steam so I would think so
Edit: I live in USA, also bought a Deus Ex key from there and made a giveaway here and had no problems.

Well if the DRM or such says Steamworks or can activate on steam then it's good. One way to be sure is to put hte title in a search then click the steam filter

Hello this is GamersGate support, how can we help you?

Yes, civ 5 uses Steam as it's drm, so all civ 5 keys will work via steam.