He'd picked a side.

He'd picked the Winchesters, humanity, and earth over the heights and bliss—the familial loyalty—of Heaven; and over further useless rebellion against their Father, who Gabriel was 97 percent certain didn't give a decomposing rat's ass about said rebellion, humanity, earth, or anything really.

He picked the only side of this crappy little apocalyptic triangle that anyone was after and that next to no one was defending. In the end it was the only side he could choose.

He's a trickster; and while that usually means he gives bullies their comeuppance after they've beaten the little guy into submission, he's not powerful enough to give Michael and Lucifer a taste of their own medicine, especially not after the fact, when one or the other would definitively be able to say they were the biggest swinging dick in the universe. In this one case, he had to jump in on behalf of the scrawny kid before-hand, and put on a show of straightforward bravery, instead of his usual manipulations. And look where it got him—dead. Didn't that just suck balls.