I recently acquired a used hot tub which the previous owner stated "it needs a circulation pump and a new heater". After replacing the pump and starting the unit the GFI tripped. Unplugging the heater allows the tub to function-plugging the heater into the control box causes the GFI to trip when the control "calls for heat". The heater checks out with resistance across the power terminals and no continuity or resistance across any other terminal combinations(ground-neutral;ground-hot). The system is 120 volts. This one has me stumped! Does anyone have some guidance for the Tinkerer. PS-a direct replacement heater is no longer available.

What amp. Breaker are you on?? Usely it!s an overload of amp. OR a short that causes one to trip. Your new heater may have a short in it;;.

I recently acquired a used hot tub which the previous owner stated "it needs a circulation pump and a new heater". After replacing the pump and starting the unit the GFI tripped. Unplugging the heater allows the tub to function-plugging the heater into the control box causes the GFI to trip when the control "calls for heat". The heater checks out with resistance across the power terminals and no continuity or resistance across any other terminal combinations(ground-neutral;ground-hot). The system is 120 volts. This one has me stumped! Does anyone have some guidance for the Tinkerer. PS-a direct replacement heater is no longer available.
Hi what brand spa is it?

Sounds as though the amp load is in excess of the ability for the heater in combination with the pump and what ever else is on the circuit. And it seems if it were much of a heater it would be 220v.

Sounds like open element leaking to ground. If replacement not available, Another may be modified to work.
On 120 volt units , have seen them use 2 120 volt circuits, 1 for heat, 1 for the rest.
Did it ever work for past owner, Is everything wired together and coming out with 1 20 Amp cord? Is it a 15, or 20 amp GFI?
Should be a Nameplate, telling required power requirements, does it have one?