He just wanted a family.

Gabriel once an archangel became a trickster because of his family. All he ever wanted was just a family; they didn't have to be perfect, after all who's family is perfect? But no one's family is what he's is and that's why he could no longer be a part of it; the trickster allowed him to break free from the bonds of what he could no longer call family because it reached the final call.

Why he chose the trickster? Truth to be told his favourite brother Lucifer taught him most of it, although he wanted to be no way connected to his family, there was just some things he wanted to hold on to.

Why he went for the Winchester's? Because they had what he wanted; nothing perfect, but a family never the less.

When he took a stand and tried to piece his family back together; his favourite brother turned around and killed him.