Possessives and Attributives

Q. Would one say, “He was a close friend of Gabriel’s” or “He was a close friend of Gabriel”? Is there a rule governing this?

A. There is! CMOS 5.49 calls your first construction a double possessive. Both forms are correct, but one or the other usually sounds more natural. “A friend of Gabriel’s” is the more common idiom.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]