If your life total is less than your starting life total, you actually gain the appropriate amount of life. Other effects that interact with life gain will interact with Resolute Archangel’s ability accordingly.

Your starting life total is the life total you began the game with. For most two-player formats, this is 20. For Two-Headed Giant, it’s the life total your team started with, usually 30. In Commander games, your starting life total is 40.