According To Luke: An Analytical Commentary

Luke 1:18-19

18 And Zacharias said unto the angel, Whereby (by what means; asking for a sign or proof) shall I know this? (demanding confirmation; contrasts with Mary’s more believing question in v. 34) for I am an old man (past natural age for fatherhood), and my wife well stricken in years (a respectful way of saying she is aged and beyond childbearing — cf. Gen. 18:11).

19 And the angel answering said unto him, I am Gabriel (means “God is my strength”; same angel in Dan. 8:16; 9:21; Luke 1:26), that stand in the presence of God (highly ranked angelic position; ministers in God’s presence); and am sent (divinely commissioned) to speak unto thee, and to shew thee (declare; reveal) these glad tidings (literally, “good news”; same root as gospel/euangelion).