Gravitational mass is the charge to which gravity couples.
Then, gravitational force is proportional to inertial mass, and the proportionality is independent of the kind of matter. Consider the famous example of a person in a falling elevator. The person floats in the middle of an elevator that is falling down a shaft. Locally, that is during any sufficiently small amount of time or over a sufficiently small space, the person falling in the elevator can make no distinction between being in the falling elevator or being in completely empty space, where there is no gravity. This happens because their paths, both toward the center of the earth, eventually converge.