I have a bit of a feeling that this isn't actually Gabriel but is Michael instead. In episode 21 it's also known that Sam is going to go through something pretty messed up and get hurt. Michael revealing himself makes sense. We also know Dean will get to play two characters. The only thing that would be able to take on an Arch Angel is an Arch angel with the Arch Angel blade. That means either Michael or Gabriel. Since "Gabriel" more resembles what we were told about Michael it fits.