It is the accepted tradition that Angel Gabriel communicated the Quran to Muhammad in pure Arabic from God. In other words, God sent down the Quran to Muhammad through Angel Gabriel in Arabic. Actually, when Muhammad first got such an experience, he thought he had been possessed by a ghost and thought of committing suicide! But it was his wife Khadija who convinced him that He had been chosen as a Prophet. This is another clear proof that Islam is not from God. Had God sent an Angel, say Gabriel, to Muhammad to convey to Quran, he would have made Gabriel convey this clearly to Muhammad instead of making Muhammad think that he had been possessed by a demon and driving him to the point of suicide. The very fact that his wife told him that He had become a Prophet and Muhammad himself thought of suicide shows that Muhammad was not chosen as God's Prophet.
But let us assume that Angel Gabriel did indeed come to Muhammad, sent down by God to reveal the Quran. The ordinary Meccans, Muhammad's own contemporaries, were almost unanimous in dismissing his revelations as anything but divine. Barring a handful of followers, not more than 70 or 80 people in 12 years of preaching from AD 610 to AD 622, everyone dismissed Muhammad's claims of receiving Divine messages. Here, if Angel Gabriel really did visit Muhammad for these 12 years, and he and God saw that people are ridiculing Muhammad and dismissing his claims, Gabriel could have easily shown Himself to all Meccans and revealed the Quran to all of them. Even one paranormal act could have been sufficient in showing that Muhammad received Divine messages from God. But Angel Gabriel showed Himself only to Muhammad and Muhammad dictated the Quran! This clearly shows that there was absolutely nothing done which could not have been done by humans. There is of course, no proof at all that Angel Gabriel visited Muhammad carrying God's messages. It could be that Muhammad genuinely believed what he felt. But if Gabriel went all the way from God to Muhammad and back, he could easily have revealed the Quran to everyone, instead of Muhammad.
The Hadith also mentions that Aisha, Muhammad's favorite wife also could not see Gabriel. Bukhari 4.440 says:
"Aisha said that the Prophet said to her "O 'Aisha' This is Gabriel and he sends his (greetings) salutations to you." 'Aisha said, "Salutations" ... and addressing the Prophet she said, "You see what I don't see.""
As we have seen Swami Dayananda Saraswati observe, the author of the Quran was not God, it was not even any learned or intelligent person. It was someone with a very limited knowledge and understanding. One who thought that God resides at a particular location in this Universe and is not Omnipresent, that God needs Angels to carry his messages.