The name Gabriel is often translated as God is mighty or the strength/power of God (where el means God); however, the most literal translation is Man-God. Thus, his name is a prophecy of the Incarnation of Christ (who is perfect God, perfect Man) whose conception Gabriel announces.
The word angel means messenger. This is one of the primary functions of angels, as seen by Gabriel giving the Virgin Mary the message that she is to give birth to the Christ. He also appears as a messenger in both the Book of Daniel (see 8:16 and 9:21) as well as the first Chapter of Luke where he appears to Zachariah, the father of St. John the Baptist (1:19; 26). Traditions in both Christianity and Judaism also credit him with the following:
- Inspiring Moses to write Genesis
- Inspiring Moses to write the Pentateuch (the first five books of the OT)
- Announcing the birth of the Virgin Mary to her parents Joachim and Anna
- Calming Joseph in a dream, telling him that Mary's pregnancy was miraculous
- Appearing to the shepherds near Bethlehem at the Nativity
- Being the mysterious young man wearing nothing but a linen garment who fled naked from Gethsemene (Mark 14:51-52)
- Announcing the Resurrection to the Myrrh-bearing Women
Gabriel, commander of the heavenly hosts,
we who are unworthy beseech you,
by your prayers encompass us beneath the wings of your immaterial glory,
and faithfully preserve us who fall down and cry to you:
"Deliver us from all harm, for you are the commander of the powers on high!" —Apolytikion of the Synaxis of the Archangel Gabriel
In Orthodox Christianity, angels are most often referred to as the heavenly bodiless powers and are considered part of the invisible creation, though they are able to manifest as visions and people — the most fantastic of which are the many-eyed Cherubim who wield flaming swords (see Gen 3:24; Eze 10:1; and Rev 4:6). During the liturgy, the faithful petition God "For an angel of peace, a faithful guide, a guardian of our souls and bodies" as well as "the protection of the honorable, bodiless powers of heaven."
Thus, one of the easiest ways to include the presence of angels in a Fantasy RPG setting would be to re-skin protection spells and magic items as angelic intervention. For example, Protection from Evil could manifest as a pair of translucent wings that envelop the one being protected.
With a little imagination, there are very few spells (Animate Dead being one) from the LL Cleric spell list that cannot easily be re-skinned with some kind of angelic special effect. Detection spells can take the form of whisperings in the ear. Remove Fear can take the form of a shield and sword wielding angel that inspires the recipient out of their fright. Offensive spells could take the effect of an angel striking down their target.
This re-skinning also implies a need to closely adhere to the will of God. If a cleric were to use one of his spells to attack a village, for example, it would make perfect sense for the spell to fail. Or, in another example, if a Hold Person spell were taken advantage of to easily kill an opponent, the spell could cease to function in order to give the victim a chance to defend itself.