I’d say it’s neither. Gravity is the observable phenomenon - something makes things fall. Newton’s gravitation law and general relativity tried to explain what gravity is, but the phenomenon exists independently of the laws or theories around it.

A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses and facts. The theory of gravitation, for instance, explains why apples fall from trees and astronauts float in space. Similarly, the theory of evolution explains why so many plants and animals—some very similar and some very different—exist on Earth now and in the past, as revealed by the fossil record.