And the angel answered and said to Mary, the Holy Spirit shall come on you, and a power of the Most High (God the Father) shall overshadow you: And for that reason the Holy Thing (God the Son) to be born will be called the Son of God.
In this verse, Gabriel accomplishes two things. The first one is obvious i.e. to tell Mary that she is going to conceive a Son. The second one is to assure Mary that God is Triune. Gabriel does this by explaining the role of each Member of the Trinity in this conception. Gabriel explained to Mary how the power of God the Father will be put forth by the Holy Spirit, as the immediate Divine agent in this work. Thus distinguishing God the Spirit from God the Father. The Holy Spirit exerted the power of God the Father as His own power – the Holy Spirit cannot exert the power of the Most High if He is not God Himself. Here it self we see two divine Beings in Gabriel’s explanation. Therefore also – Not only is Jesus Christ God from eternity, but on the special way he is conceived he shall be called the Son of God. Also note how Jesus is called the Holy Thing which is Gender neutral.
Mary doesn’t seem to have a problem accepting what Gabriel said. This can also be seen when she sang the “Magnificat” (Luke 1:46-55) praising the individual work of all the Members of the Trinity. The song of Mary derives most of its words from many versus in the Old Testament and still it flows perfectly (inspired by the Holy Spirit).
And Mary said:
“My soul glorifies the Lord and my spirit rejoices in God my Savior (God the Son), for he has been mindful of the humble state of his servant.
From now on all generations will call me blessed, for the Mighty One (God the Father) has done great things for me—Holy is His name. His mercy extends to those who fear him, from generation to generation. He has performed mighty deeds with His arm;
He (God the Holy Spirit) has scattered those who are proud in their inmost thoughts. He has brought down rulers from their thrones but has lifted up the humble. He has filled the hungry with good things but has sent the rich away empty.
He (God the Son) has helped his servant Israel, remembering to be merciful to Abraham and his descendants forever, even as He said to our fathers.”