Okay, years ago I read this AU fanfiction where Gabriel I believe he was a lawyer and works with Sam and though being friends with Sam he meets Dean. The fic was from Deans' point of view I think. Some of the things that stood out were that Gabriel has Bipolar Disorder and was born in some messed up cult and a plot point was meeting his half brother Castiel again. There was also a really emotional scene where Dean and Gabriel fight and Gabriel goes into a depressed stage.
I really want to find it and have been having problems so please if you know it leave a post please!