I understand that the Feast of the Immaculate Conception (December 8) is the celebration of Mary being conceived immaculately. Can you please explain why the Gospel reading for that day’s Mass refers to the Archangel Gabriel coming to her when she conceived Christ immaculately?
Yes, the Feast of the Immaculate Conception celebrates Mary’s own conception free from original sin; and, yes, the Gospel reading for that feast also refers to Christ’s own conception. Christ, being God, was also free from original sin, but since he did not need to be preserved free from original sin (i.e., saved from sin) his conception is called the Incarnation or the Virgin Birth; it is not usually referred to as an immaculate conception.
The Gospel reading (Luke 1:26-38) is the story of Gabriel’s Annunciation of Christ’s Incarnation because within the story Gabriel refers to Mary as “full of grace” (v. 28, Douay-Rheims), an implication that she was free from all stain of sin. Because one of the purposes of the Immaculate Conception was to prepare Mary for her vocation to be the Mother of God, it is fitting that the Gospel reading be the call of Mary to be God’s Mother and her consent to that call.