Well, pool repair guy came out and said there is a small amount of water inside the light assembly. Dosen't know if the gasket is leaking or the assembly is cracked. Reccomends replacing entire assemly instead of just the bulb (which would come with a new gasket). Said the reason it only tripped on 100% was because any other setting did not draw enough amps to trip the gfci. Just thought this info might be helpful to someone else with a similar problem.