The word angel, in both Hebrew and Greek, means a messenger, and the chief function of angels in the scriptures is to bring God's message to humans. Although they are of a higher order of creation than Man, they are servants of man on God's behalf. Gabriel is a special angel, one of only two who is ever named. Gabriel appears only elsewhere in the book of Daniel. That's why it's Gabriel who comes to Zechariah, and to Mary, because the seventy sevens are completed, and all those things promised to Daniel are about to take place.
Sin is to be dealt with.
The most holy one, the Messiah, is about to be anointed. Behold, I will send my messenger, who will prepare the way before me. This was to be the ministry of John the Baptist, Zechariah and Elizabeths' son. There are two equal and opposite errors people fall into regarding John. The first is to regard him too highly.