Robbie didn't write this song, he covered it, the song in most parts was written by an irish song writer called ray heffernan, he played the song to robbie after meeting him in a pub and robbie along with guy chambers turned what had already been written into the song we now know
The song is comparing a woman to an angel, in that like an angel she protects him and stands by him through anything. Like when people are referred to as someone's "guardian angel" for saving them. But Robbie, who wrote it, says it's about his aunt and uncle, so that's the original intent.
As for Gregorian, they did an album of love songs and ballads. I don't think they're even monks, they just seem to REALLY like this style. I don't blame them, most Christian music is beautiful.
