In Cecil’s article What’s the deal with angels? - The Straight Dope, in the subheading about Gabriel he says,
“There is some confusion about Gabriel and the Holy Ghost in Christian writings, since in Matthew 1:20, the Holy Ghost gets Mary with child, while in Luke 1:26, it is Gabriel who “came in unto her.””
However, Luke is very clear about the way Mary conceived, and it is definitely not by Gabriel. I will be using the American Standard Version to show this, although any other should work just as well.
Starting in Luke 1:26 it says,
26 Now in the sixth month the angel Gabriel was sent from God unto a city of Galilee, named Nazareth,
27 to a virgin betrothed to a man whose name was Joseph, of the house of David; and the virgin’s name was Mary.
28 And he came in unto her, and said, Hail, thou that art highly favored, the Lord is with thee.
If someone stops there it would be possible to think that Gabriel had sex with Mary, but if you read down just a few verses it says,
34 And Mary said unto the angel, How shall this be, seeing I know not a man?
Mary certainly would not be asking this if she had just had sex with the angel. Gabriel then goes on to describe how her conception would occur,
35 And the angel answered and said unto her, The Holy Spirit shall come upon thee, and the power of the Most High shall overshadow thee: wherefore also the holy thing which is begotten shall be called the Son of God.
This is in line with the much shorter description of Mary’s conception in Matthew 1:20. God’s word is true.