It is tricky but when you get it you will always get it.
I always go up the stairs next to the room buttons. Move the camera, not Gabriel, around to the corner where you want him. You should see Mosley leave his room, he can't see you. When he turns the corner you go almost to the corner and click on the floor then click escape. Gabriel will be there.
Look around the corner and Mosley will be unwrapping the candy. Right away click on his pocket. Thats it. Good luck,