Steam/GamersGate question

Pretty sure it was 50% off with another 35% off yesterday at GreenManGaming. Not sure about GamersGate, but GMG will give you a key to activate on Steam.

If you want more than 33% off, wait another week and a bit for Winter Sales.

I'm pretty sure you can get it cheaper on GMG with one of their vouchers and that gives you a key for Steam. Otherwise, it'll say under the DRM section on GamersGate. If it says Steamworks, then yes.

In general, other DD's will say that the DRM is Steamworks or that you need a Steam account to activate the game. In those cases, they're usually safe to buy.
Otherwise, search google for your digital distributor in question, along with "activate on steam" and (for a lot of them) you'll come to a thread on the steam forums with lists of what games activate on steam after being purchased elsewhere (these are not always up-to-date; if the DD page says "Steamworks" or "requires Steam account" trust them! - If they lie, you can complain to customer service)
As a side note, you can check out e.g. cheapshark.com (I think) and several other, similar, sites which look at DD sites and finds the cheapest prices (and most of them you can check that you only want to see steamworks games).