To see if your GFI is working correctly, simply press the 'test' button. If the other button pops out and the power is shut off to the spa, then the GFI is working normally. Push the 'reset' button to get things going again. The red light could not have been flashing if there was no power to the tub.
The red light flashes when the heater gets too hot. This is normally caused by a dirty spa filter. Shut off the power to the tub, remove and clean the filter, replace and turn the power back on. Should be fine.