Gabriel didn't call himself a Trickster for nothing.
Yes, Lucifer had taught him most of his tricks, but what Gabriel's brother did not realize was that Gabriel had taught himself a few new tricks as well.
Gabriel was unsure of his brother's power, and so he had thrown himself into the game, standing up for himself and yet coiling tricks and plan Bs and plan Zs up his celestial sleeve. Lucifer may be the master of deceit, but Gabriel was the one with quick fingers and layers upon layers of plans, many of which involve fleeing. Hiding.
Gabriel was tired of hiding.
He had brooded for a long time after Dean had told him that he was too afraid to stand up to his family. Gabriel had brushed the words off at first. He didn't want to kill his brothers. He didn't want to kill Lucifer or Michael or even little Castiel. He had left because he couldn't bear watching his family tear each other apart. Not because he was too afraid to do anything.
He had stayed out of the fight. He hadn't wanted to side with Michael or with Lucifer. He hadn't want to be heaven's good little guard dog or the servant of the devil. He hadn't wanted his brothers to fight, still didn't want them to fight, but they were going to anyway so he had set himself up a little witness protection program and left heaven.
He had avoided the ridiculous spat Michael and Lucifer insisted on having because he loved his brothers too much to side with either, loved them too much to watch them murder each other. It wasn't because he was afraid.
Gabriel had turned to the only alternative he thought he had left-let his brothers fight it out and hope that they didn't destroy the earth. He drew Dean and Sam Winchester into his game, attempting to convince them to say yes and become the vessels his brothers needed to end this infernal spat.
Castiel had dragged himself into it, determined to save the Winchesters. This little brother of Gabriel was falling, but not so he could side with Lucifer. He was falling for his father's creations.
Gabriel suddenly saw another option.
He could fight for humanity, despite their messy, sinful lives. They were intricate and flawed. They were full of fear and hope. Neither Lucifer nor Michael liked God's beloved creations. Lucifer would wipe them from existence; Michael would trample them under his feet. Gabriel didn't want that. He had walked among humans for millennia, had seen how hard they tried to make their short hopeless lives count. They tried so hard. He could fight for what his father created; he could fight for what he wanted to fight for. Damn the consequences. Dean was right. He was too afraid, too afraid to stand up to his brothers, to tell them what he thought of their actions. Lucifer and Michael weren't the only angels in the family.
He was Gabriel, the Trickster, Loki, the archangel who walked among his father's creations and liked what his father had made.
Gabriel chose a side.
He expected to die when he faced Lucifer. He did die. But Gabriel didn't call himself a Trickster for nothing. The archangel always had a plan b. He just hoped the Winchesters wouldn't notice the candy wrappers littering his resurrected wake.