If that is a different GCFI circuit, then I would say you have a problem in the wiring or element.
You could take the back off and disconnect power from element. Tape over the ends of power lines and plug into GCFI circuit. If GCFI doesn't trip while element is disconnected, then I would guess element is leaking current through ground even if it glows solid red and shows a good Ohm reading.
If GCFI does trip with element disconnected, then it is probably the cord.