In the end all the Angels die(except maybe Castiel) and are reborn as humans in the 20th century in the mortal forms they died in.
Few of them remember bits of their old lives and Gabriel is one of them.
He goes to find his remaining angel brother Castiel or to find Sam and Dean to reassure himself that he isn't crazy.
Bonus points for him having squabbling older brothers named Luc(Lucifer) and Mikey(Micheal).
Other bonus points for Anna, Uriel, Zach and Raphiel being talked about.