Welcome to TFP.. A Great resource for all your pool related questions...
Well, I know nothing about the Fiberstars system, but the first thing I would do is to remove the bulb and try it again. If the GFCI does not pop with the bulb removed then it is the most likely cause.
If the GFCI pops with the bulb removed then I'd be looking at the wiring or inside a junction box if you have one.
Remember a gfci is ground fault. It only takes one wire grounded and not shorted. Gfci trip with milliamperes difference between hot and neutral so looking for arc marks or burnt areas won't find the problem. You have to look at all connections and if you don't find the issue you have to look at the entire wire through the system
Since it's a FiberStar system, all the electrical parts are in the light box head. I think those lamps are metal halide, so they have a ballast. It could be as simple as moisture getting into the box, or it could be the ballast going bad.
Turn the power off and open the box. Don't touch the lamp with your hands. You don't wan to get oils on the lamp.
Look for any moisture in the box and on any electrical equipment in the box. If you find any moisture, dry it off and close the box and try it again.
If you don't find any evidence of moisture, you may need to troubleshoot it. There is a troubleshooting guide in the manual. If you don't have one, you can download one for it.