I see you have 3 questions and not just one as you stated

here are the answers:-1- Prophet Muhammad believed in Prophet Abraham's message; monotheism which is Islam.2- Yes it was the Archangle Gabriel who conveyed God's Book the Holy Quran to Prophet Muhammad3- Gabriel conveyed all the other Books of God to all God's Prophets.God ordained only one religion which is Islam or monotheism , meaning ‎worshiping Him alone without associating anyone or anything else with ‎Him‎ ‎Ever since He created Adam , God sent some Book to people by His ‎Prophets so that people knew the purpose of their existence on earth, which ‎is to worship Him, and to let them know that they will be judged after death ‎when the DAY of JUDGEMENT arrives where everyone will be rewarded ‎by going to Heaven if he obeyed God, or he will be punished by going to ‎HELL if he disobeyed God‏.‏‎ ‎All Gods messengers conveyed the same message which is Islam meaning ‎worshiping the ONE & ONLY TRUE GOD , therefore all prophets ‎including Jesus PBUH were Muslims‏.‏All Gods Books contained the same basic teachingsThe difference is only in the lawsGod did not reserve any of those Books except the HOLY QURAN , for He ‎knew that they were temporary , and for specific people at specific times‏.‏We now have the HOLY QURAN . It Is for all mankind and contains all ‎what was in the old books. Through it we can understand the Creator and ‎everything we need to know‏.‏‎ ‎I mean that Islam is GOD'S ONLY ACCEPTED RELIGION and ‎MUHAMMADS message is NOT a new religion as most people think‏.‏‎ ‎