One of the things I put a question mark with regarding Islam, If I can be as bold to do so, is the use of the Angel Gabriel as the messenger from Allah to the Prophet.

We have a being mentioned in Daniel (Old Testament) which makes him important in the Jewish religion. (around 500 BC)
Then the same angel appears to some people foretelling the birth of John the Baptist and Jesus.
Interestingly this angel names Jesus, the Son of God. (Around year 0)
Then this same being appears and gives unto the Prophet the message of Allah recorded in the Qur'an. (finished around 632 AD.)

Three religions having the same messenger. Something is up with this.