I fought something similar to this for a whole season. Turns out the outlet itself had a little corrosion where the ground pin goes in. Swapped out the outlet and everything worked. The strange thing about it was that it wouldnt *always* trip, and when it did, it would run for very different amounts of time before doing so (pump could run for 2 minutes, 2 hours, or 2 days before it acted up again).

Check the outlet to see if you have any corrosion. Also check the green bonding wire to your pump, along with the other electrical connections to the equipment. Since the GFCI basically requires every last bit of juice going out to actually come back (my simple way of putting it), any small amount of corrosion, loose fittings, etc. can cause it to trip.

-Chris