Circuit problem in bathroom
GFCI's trip for a specific reason, that's when there is an imbalance between the current flow of the hot wire of a branch circuit and the neutral of the same circuit. So it's either the fixture itself has some sort of leakage or the switch leg leading to the fixture or the before mentioned you have the switch wired incorrectly. I would try to disconnect the fixture from the circuit and see if the GFCI trips. If it doesn't it's the fixture, replace it. If it does trip it's either the branch wiring or the switch. Be sure and wire nut off the exposed wires while testing without a fixture.

Good luck and stay safe.