IF GFCI still trips the follow are other
Since it wasn't the heater, the remaining possibilities are few and rare.
First, look for water leaking onto the equipment.
Second is disconnecting pieces
of equipment beginning with ozonator.
Third and rare is the pcboard itself
If not any of those, then consider the gfci or power to spa is compromise. To check,
disconnect the wires from the incoming control box from the gfci. Make sure the
hots and neutral are not touching anything. Then turn on power. If the gfci
still pops, either the gfci or the line is shorted. See an electrician.
If the gfci doesn't pop, the failure is likely the pcboard itself or the
transformer. Both of these failures popping the gfci is rare.