I have the same tub and had a similar issue. We originally connected a 12guage with 20 amp circuit breaker and breaker kept trippin. The solution was to connect 10guage wire with a 30 amp breaker. Connect that to a gfi then connect the 12 guage wire coming out of the generator to the gfi (woulda made sense to just have 10g coming out but it was made in China). Being that theres only a short lead coming out of the generator its fine to run 10gauge, just make sure there connected seperately to a gfci breaker. The pump is fine 12 guage 20 amp.

See if the unit has any information about the power consumption. If it's given in watts, then watts/voltage = amps.

If given in amps, then it's probably either 15 or 20 amp, and either 120 or 240 single phase.

Minimum wire size for a residential appliance is #14 awg, but you can use #12 awg. It's always safer to use the larger size wire.