I have a deal to get Dishonored on Green Man Gaming for 48 dollars. I'm not familiar with this site. Will I be able to transfer to steam? Is it trustworthy? Is it a good enough deal or would I save significantly more by waiting for the December sale on steam (I know no one knows the answer to that, but you can guess better than me because I've only been on one steam sale).

I would advise not to. These deals tend to be legit for the most part but there are also a lot of scammers. It's your call, I don't have a lot of experience with these things but I have seen people suffer for it.

Many games can't be transferred to Steam, even if you can buy them there. My copy of Portal 2 shows up as a Steam game despite being purchased from Amazon.com, but my copy of Hunted: The Demon's Forge does not.