We need to make sure the GFCI is the problem.  99% of the time it is not the problem.

The way to do that is to test the parts of the spa individually that normally cause real ground faults.

The unit is repairable as well and I have the parts at a discount for you. 

There is no better GFCI around and normally if it is tripping, there is a ground fault in the system somewhere.

The wiring from the spa to the box needs to be tested as well for any ground leaks.

The process of testing the spa for ground fault is to.

Always make sure the main breaker is off any time your are working in side of the control box, making changes. 

1/ Disconnect both leads off the heater from the circuit board.
Do not take them off the heater element, because if it is not the problem you  can damage the electrodes by twisting them.  Place the wires where they don't touch anything.

2/ Turn the power back on and see if the spa runs normally.   If the spa runs nomrally and doesn't trip the GFCI, then the heater element needs to be replaced. 

Next test is the blower.
Always turn off the power at the main breaker to the spa.

1/ Replace the heater wires on the board. Don't over tighten them. Just snug is good. 

2/ On the side of the control box, determine which cord plug has the blower attached to it.  It is often labeled but not always.  You can trace the wire from the blower back to the board and unplug it.

3/ Turn the power back on and see if the rest of the spa runs normally.  If it runs OK then the blower has the ground fault. 

4/ You cant see if the blower is wet and do a blower run to dry it by (main power OFF!) removing the green ground wire off the circuit board, plug in the blower (turn power back on) and run it for 20 minutes.  

5/ (Main power off) Put the green blower ground wire back on the circuit board connector and (main power back on) run the blower to see if it trips the GFCI.   If it trips the GFCI after this test then it may need to be replaced.  You can try another 20 minute dry out and see if it does it again. (step 4-5)

If the GFCI still TRIPS after these tests then you need to unplug the remaining devices from the spa one at a time and reset the GFCI until the GFCI stops tripping and "holds".   The one that you  unplug last and the GFCI holds is the "culpret" and it needs to be replaced or fixed.  

Unplug the pumps, ozone generator, and any device left plugged in "one at a time" to test this.  You have to do it one device at a time or you will get confused and it takes longer. 

NOW if the GFCI still trips after all this testing, then we need to see if the wires in the conduit from the GFCI to the spa are where the ground fault is.

1/ To test this, you remove the wires from the GFCI going to the spa and reset the GFCI.  If the GFCI holds with no load attached, then I would run a separate test wire from the GFCI to the spa and try it.  

2/ Use a section of 6-3 w ground sheathed cable also called "Romex"  and hook it to the GFCI and then to the "Line In" power connections on the spa.  (You cannot leave the spa this way as it is only for testing!) . 

3/ If the  GFCI holds and the spa runs OK, then the wires going to the spa have a ground leak and are the cause.  (I have found this to be the case, several times.)  

4/ If the wires are ground leaking they are dangerous and need to be replaced with clean dry conduit and clean dry wires.  
There should be no possible way for water to enter the conduit.