Gabriel doesn’t come to Mary under his own power or by his own decision. He is sent, rather, to Mary by God. God sends Gabriel to a specific town, to a specific girl betrothed to a specific “man of David’s lineage.”
God doesn’t look down through time and see a young girl He knows will choose Him and then say to Himself, “Well, she’ll do.” Rather, He chooses Mary. She, by His choice, becomes the woman blessed above all women by God Himself.
“When the sixth month came, God sent the angel Gabriel to a city of Galilee called Nazareth, where a virgin dwelt, betrothed to a man of David’s lineage; his name was Joseph, and the virgin’s name was Mary. Into her presence the angel came, and said, Hail, thou who art full of grace; the Lord is with thee; blessed art thou among women.”
Mary does not resist God’s call. She does question Gabriel, asks ‘how can this be?’ But God’s call is irrevocable as the author of Hebrews tells us. And Isaias (Isaiah) muses:
“What a strange thought is this! As well might clay scheme against the potter; handicraft disown its craftsman, or thing of art call the artist fool.” (Isaias 29: 16)
So, Mary is the Mother of God from before time.