The Greek word used for "full of grace" is the word "kecharitomene," which means to be "completely, perfectly, enduringly endowed with grace." (Blass and DeBrunner, Greek Grammar of the New Testament). It is the only time that this word is ever used in the Bible, and it is used in reference to Mary. But that is not all. Another interesting thing is that "kecharitomene" is a perfect passive participle, which "denotes the present state resultant upon a past action" (New Testament Greek for Beginners). What does that mean? It means that Mary's state of being full of grace came before the angel Gabriel called her full of grace.
Now, what does all of this mean to us laypeople? It means that Mary was ALREADY completely, perfectly, and enduringly endowed with grace BEFORE the angel Gabriel annouced to her that she would give birth to Jesus, BEFORE Jesus was conceived, BEFORE Jesus was born, BEFORE He performed His first miracle, BEFORE His death on the cross, BEFORE His resurrection from the dead, and BEFORE His ascension into heaven. In other words, even before Jesus was born, God, in His goodness and mercy, had already granted to Mary the fullness of grace and salvation. She did not have to wait until Jesus died and was resurrected to be saved. She was already given grace and salvation before the angel Gabriel first appeared to her. This is why Mary could proclaim in Luke 1:46-47,"My soul proclaims the greatness of the Lord; my spirit rejoices in God my SAVIOR."
We see that, according to the Bible, Mary was completely, perfectly, and enduringly endowed with grace before the angel Gabriel appeared to her. How long before? The Bible doesn't say. But according to early Church tradition and belief, Mary was full of grace from the moment of conception. This is where the Catholic Church gets the doctrine of the immaculate conception of Mary, which teaches that Mary, from the moment of conception, was free from the stain of Original Sin. God granted her this grace because she was to bear the very Son of God within her. It is because of this grace that all generations call Mary blessed (Luke 1:48).
So when you hear or read the story of the angel Gabriel coming to Mary to annouce that she will give birth to the Son of God, don't simply skip over the part when he greets Mary as full of grace. Instead, think about what that greeting truly means, and join with Elizabeth in saying to Mary, "Most blessed are you among women, and blessed is the fruit of your womb" (Luke 1:42)!