My friend is wiring a hot tub. He has a 40 AMP at the house and ran 8/3 from the house to the receptacle at the tub. He grounded the ground at the ground bar in the panel, and connected the like color wires directly to the tub at the recetacle box, 3 1/2 feet up from the floor. The GFI (a 50 AMP) trips, although the green ground wire is connected to the pump motor. Should there be another ground somewhere?

All Green/Grounds need to be tied together. Neutral(whites) need to be isolated from ground. Unplug or disconnect pump motor, and if it doesn't trip, then motor is leaking to ground.(repair or replace) Test for other items also, blower etc.

Thank you, will give that a try.