Sorry, I'm new to GOG and Hotline Miami was the 1st game I've bought from here. I'm wondering if there's any way to sync this copy with Steam. I know with purchases on the Humble Store, you're able to get a direct download and a Steam copy. Can somebody tell me if that's possible with this? Sorry if this is in the wrong section or has already been answered.

I'm sorry, that's not possible. GOG and Steam operate completely separate, neither of them gives any keys for the other.
In rare occasions, developers gave GOG customers Steam keys, so contacting them might be your only chance.

On rare occasions the developers give out keys. For example, when I purchased "Race the Sun" on GOG. the developers, "Flippfly", gave me two keys so I could get the game both from their website and steam. But yes, Steam and GOG do not share games or buying habits. so what you by on GOG will not effect the games Steam recommends.