Will the heatpump on the Softub turn on without water in the tub? I drained the tub for the summer and tried to test the pump and it tripped the GFI

Easy way to test for the heater issue is to disconnect the heater from the board. Disconnect both leads from board and let the spa call for heat. There is also a chance the GFI breaker is bad however most times the heater fails and the breaker is doing its job.

if your GFI trips soon after the spa goes through the purge cycle then you need a new heater.
Yes the heater can develop a dead short and it causes the GFI to trip when power is sent to the heater.
Rod @ US Parts Center .com

#1. DO NOT GET INTO TUB FULL OF WATER WITH A GROUND FAULT.
2. There is a short. You need to get multimeter and start checking each part for current running to ground wire.
3) Power is ON. Connect power without GFCI.
4) Tape tester leads to wood sticks to keep hands away from power.
5) Do not touch anything directly or you will become the ground rod.
6) Ground wire must be present, and bonded back to main panel box.
7) Do not sit or touch directly on cement or bare ground. Do not touch or lean into anything made of metal.
8) Body and parts must be on sheet of plywood, or similar non-conductive dry surface.
9) Test each part with spa, but also test electric line coming from main box.
10) GFCI will also trip with any fault detected on line. So the wire coming to spa may have the fault.
11) Add a new dedicated line from main box to spa.

http://waterheatertimer.org/How-to-wire-GFCI.html
http://waterheatertimer.org/See-inside-main-breaker-box.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Did you try an electric motor rebuilder? $500 seems too high to repair a motor. Motors have either a capacitor or dual windings to get them started and it sounds like yours has failed. Get the numbers off the MOTOR and call a motor rebuilder in your area for pricing.
Gary

If you ran it without water. It's trashed. No pump or heater is ever to be tested dry!

Unplug tub from GFI, does GFI hold, plug something else into GFI to see if it holds under load. If all is ok then make sure you have a good ground from the motor to the GFI and the polarity is good. If 120 vac black wire to gold post, white wire to silver post and ground to green. Check proper connections, including polarity and ground at tub junction box and at pump motor it self.

Either the GFI is bad or it is tripping because there is a short in the system (obviously, right). I would turn off your panel break, disconnect the GFI and using a meter test to see if there is a short in the system. You would check the hot lead coming from the GFI to the tub and see if it is shorting. If you can't find a short, you should try replacing the GFI. If there is a short then you need to search for the short. Hope this helps and good luck!

Open your drain valve under the unit to break any air lock that will have gotten there when you drained the tub.

I ran into this problem with a washer this summer. the gfi breaker needs to be replaced. it is weak and trips easily. if you can run an extension cord to an outlet without the gfi andrun your washer, you probably will not have any issues. that is what I found with the washer this summer. when it was plugged into the gfi, itwould trip. but when I had it plugged into a regular outlet, it was fine. Joe