Watch over humanity. Occasionally carry out judicious miracles and tasks to try and steer humanity to do good. Prepare for the end of days. Though we did rather avoid that last one. Gabriel did have that rather large role with Mary that seems quite incongruous. He certainly wouldn't have been my first choice. Sometimes I wonder if God was having a bit of fun with that one.
Oh, and personally, since Crowley and I made our Arrangement, I did carry out the odd temptation on his behalf. That was fine though, because he did a few miracles on my behalf.