GFCI receptacles and breakers trip because of an imbalance between the ungrounded and grounded conductors. How old is the ballast and light fixture? Water your ground rod outside by your service. If your neutral starts floating this could be your problem. Is there anything else that shares the same circuit with your lamp? And it doesn't have to be on the same GFCI. If it was your ballast it should trip on the start up.