Gabriel and Castiel were very close in heaven (Gabriel was very [platonically] attached to Cas and his innocent nature.) After God gives Gabriel a message about the person who will complete him (Sam), Gabriel grows increasingly depressed and desperate. After God gives him a similar message for Castiel about Dean, Gabriel refuses to deliver it and flees heaven.
There's something about Gabriel creating the perfect vessel for Cas to occupy on earth by collecting things like gems for his eyes.
I believe in this fic Sam, Dean, and Chuck all went to school together and stayed best friends. Also, Dean lives with Ellen and Jo, but didn't know they were hunters.
EDIT: Thank you, malcolm_doyle! Fic found in comments.
This should be enough to identify the story by if anyone knows it, I don't want to spoil too badly because it really was a great read. Last I read, it was a WIP, but it may be complete now.
Thank you so much if you know what it is!