You're right, I just don't know about how he received the Koran. Either he made the whole thing up (difficult to do I would think since he was an illiterate caravan hijacker...a warlord thief and not a scholar) and dictated the work from his own imagination, basing it on what he'd already heard about Christianity and Judaism. Christianity was thriving in the middles east at this time. Ancient Churches were solidly established at Memphis, Antioch, Philadelphia, Yemen and many other places. They had already received the Gospel and had no need for a 'new' and 'final' prophet.
The other possibility is that an angel DID appear to Mo, but it wasn't Gabriel. It was Lucifer.