Gabriel doesn't interrupt the dream until the memory is over. He doesn't have the option to, he's just an observer, but even if he could he wouldn't. Obviously this is something he hasn't seen before - he remembers it more clearly than most humans would remember their birth, but not by much. Once it has played through, he's not sure what to say or do. He stays where he is, standing at a distance and watching, expression carefully neutral. He's silent for a few moments, but after a while he decides to make his presence known - or rather to announce it, since he'd be a little surprised if Raphael were unaware of him.

"Is that really how it happened?"