In a deleted scene, disposable demon asks to hit Aziraphale during the trial. And it's not Gabriel who gives it permission, but he doesn't deny it either, so... Well, as a deleted scene maybe we shouldn't even consider it part of the canon (or yes, why not?), but I think it's a mountain that we must also climb.

Ok, let's remember one of the things that Gabriel says about Aziraphale at a given moment: "he is not behaving like an angel." Gabriel certainly doesn't do it now either (I won't be the one to defend him at this point) but, why is it completely forgivable in Azi's case and not in Gabriel's case?

And on the other hand, I think Gabriel feels that here, the betrayed are him, as his boss, and together with his entire species, as angels. He feels that they are the victims of a being whom he no longer recognizes as either a subordinate or a member of his lot. And it's also easy to assume that he no longer regards Aziraphale as someone he has an obligation to protect. Because Gabriel simply doesn't know what Aziraphale is anymore.

However, I don't think he's really liking any of this. It is easy to see how upset he is at what is happening...