Agreed. It is generally recognized that Mohammed cobbled his new religion together from information about Christianity he received from a relative of his wife (who was a monk belonging to a heretical sect), some things he knew about Judaism, and native pagan beliefs.
Actually, the tradition among Arab Christians, which recently got scholarly support from the fact that sections of the Qu’ran are nonsense as Arabic, but perfectly intelligible East Syriac, is that Mohammed started as a missionary from the Church of the East (the Nestorian church centered in what is now Iraq and Iran) then fell prey to demonic delusion and set himself up as a warlord.
Interestingly, one of the Nestorian bishops at the time Mohammed began his career was named Gabriel (I think he was the bishop of what is now Kirkuk), and there is a tradition of some bishops bearing the title “Angel” (cf. the Greek Orthodox Bishop of Bosra-Haran in Syria, who still bears the ancient title “the Angel of Haran”, and the style of address to the bishops of Asia Minor used in the Apocalypse of St. John). It is possible that before he fell into delusion, Mohammed reporting that “the Angel Gabriel said...” was simply reporting instructions from his bishop.