If you know that the GFCI is good, then the heater that you are plugging could have a "hot to frame" ground fault condition. This means that the ground prong on the plug is carrying some current as well as (or instead of) the neutral. A GFCI measures the current coming in from the hot and compares it to the current going out on the neutral. If they differ from each other, the GFCI will trip.
In your case the heater could probably have a "hot to frame" short - also known as a ground fault - and some of the current is going onto the ground wire. Either find the fault by taking the heater apart, or buy a new one and toss the faulty one before it causes a serious problem.
As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS
WORK SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.
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