Sam
Why not? I think it's a fair comparison. They're both not human, so they have a completely different set of moral / social standards than we do. They both were on the team that wanted the apocalypse to happen. They both changed their minds on that because someone showed them why they should.
Didn't Ruby ever do things from her old demon days when she was first starting out on the 'being good' thing? Maybe still does? It's not like it would have been fair to expect her to suddenly stop acting like a demon if she's been one for... however long she's been one, right? So why should we expect Gabriel to stop acting like he always has right away?
Like I said, I don't expect you to trust him, or like him, or ever forgive him for what he did. I just need you to give him a chance.