If i buy a game from GreenManGaming.com will it work on Steam?

i never bought a PC game from 3rd party websites, i just buy from Steam, which is why i ask such a noob question lol.

i wanted to get the game "To the Moon" on GreenManGaming (since thats the cheapest place i seen it) and wanted to know will it work on steam if i prchase it?

I have never used GMG, but it most likely doesn't since there isn't any mention of it. If you check something like Black Ops they specifically say you need a Steam account to play.

All of the games I bought from them have.

Here is a thread on the Steam forums with a list of games that you can activate on Steam. There is one for Gamersgate as well if you are interested.

I bought Alan Wake from there and they just gave me a CD key for steam. But not all games do that from there, some of them have to be downloaded with their downloader "capsule" and played through that

4 out of the 5 things ive bought fomr GMG worked on steam.

One way to (kinda) check is search Steam Store for the game. If it's not there, chances are you can't have a Steam version. Though this is not always true (Resonance for example.).
And Greenmangaming typically states that the game requires you to have Steam if it comes with a Steam key.