Why didn't Gavin know......(spoilers)

I got the impression that Sasha might have really died and Gavin brought her back to life. That would mean the story she told her mom was bs. But even if what she told her mom was true, Gavin might have known anyway - just because he seems surprised about something doesn't mean he really is.

I think there's room to believe that immortals are imperfect by being human. Perhaps, Gavin only had a limited range of knowing things and that's why he kept an iron grip of those who dealt with as they were his extended eyes for certain things.