if i buy a game from greenmangaming does it give me a steam key?

Not all games. If its for steam its written in the description, "Third party DRM : steam" or something.

If it says steam required you get one

Only if it says:

"Third party DRM: Steam
This game requires a free Steam account to play."

...on the store page. It'll say just under the screenshots if so. Example.

Depends if it says

Third party DRM: Steam

This game requires a free Steam account to play.

Depending on the game, you either get a steam key or you download it directly from GMG through capsule

Its usually downloadable games, some run stand alone, some require their special software like Steam called Capsule and some will be Steam CD Keys. Look for the ones that say in the game description "This game requires a free Steam account to play.".

Also, if you do buy right now, don't forget the following 25% off Voucher at checkout.

GMG25-EVFWS-4Z4ZN

No retail, it's all digital.