Sounds like your welding outlet is 220 only. the 3 wires being 2 hots and an Equipment ground. If your using 220 only for your heater elements then you wont need a neutral for them. **Wire the white pigtail from the GFCI breaker to the equipment ground, BUT DO NOT use the neutral lug on the GFCI breaker for anything** this will allow the 'test button' on the GFCI to still work properly, and protect your 220v circuit.
Your control Circuit is going to be were you need 120v. instead of trying to jerry rig a neutral and creating more problems, why not run a second extension cord you plug into a 120volt GFCI outlet, and use that for your control circuit and pumps.
As to the OP, and some one saying he should replace the house panel breaker with the GFCI, that would create a bigger problem in his case. The house breaker is 30 amps, which means the wiring is most likely 10awg, the GFCI breaker in a Spa panel is 50 amps. He would have to replace the wire with 8awg or buy a 30amp GFCI breaker.
While that would be ideal, simply locating the Spa panel right next to the dryer outlet, and running a longer cord to the brewery control panel would adequately protect the 'wet zone'.
I made an electrical safety thread a while ago, that might help some.
