Gulum is an example of

Gulum is a chordophone because it uses vibrating strings to make sound. Players pluck or strum the strings.

Membranophones use stretched skin like drums. Idiophones are struck solid objects. Aerophones use vibrating air. Gulum uses none of these.

Guitars, harps, and lutes are also chordophones. Any instrument with strings belongs to this family.