Gabriel was acknowledging the grace God had filled her with at her Immaculate Conception. And if she was filled with grace at the age one is able to conceive, then she must have been filled with grace and free from sin up until that point. How did she accomplish that if she were still in need of a savior at that point.
She certainly needed a savior like all of us, and she was saved by Christ as we all are, but God granted her the grace of drawing on the grace Christ gained for us when he died on the cross. It is the same way the saints of the OT drew on the grace of God. Not by the sacrifices of the Temple, which, as Paul told us, had no power to erase sin, but through the looking forward to the sacrifice of Christ.
Christ’s sacrifice is eternal, not bound by the limits of time. You are thinking two dimensionally, and as a creature living within time. God’s essence doesn’t depend on the passage of time, rather he is outside and inside it at the same time.
Mary did not commit any actual sin because she was cleansed from the stain of original sin at the moment of her conception. While we are cleansed of the stain of original sin and actual sin at the moment of our baptism. Mary, however, being full of grace was also free of concupiscence. She never had the desire, the inclination to sin, which is why she was still full of grace when Gabriel appeared to her to announce that she was to be the mother of the Messiah.