First check to make sure the GFCI is not actually detecting a ground fault. Check for shorts between the fan motors and cases of your appliances, etc. Too much moisture can activate these devices, also.

To calculate the total wattage you are drawing, add together the wattages of all appliances, including fans. To convert fan horsepower into watts multiply horsepower by 780. Therefore, a 1/4 horsepower fan will take about 200 watts.

Here is an example:

2 heaters @ 1500 watts each = 3000 watts
2 fans @ 1/4 hp. each = 400 watts

Now divide by the line voltage to get the total current. The typical US line voltage is 115 volts AC. 3400/115 = 29 Amps. This draw will, of course, pop a 20 Amp circuit breaker.

BTW, your 12 gauge cord can safely carry only 20 Amps. The circuit may not fail right away as the thermostats on your heaters may not cause them to both run at the same time, but when they do, your power consumption will reach a peak. If your line voltage is higher, say 120 V. as it is around here, you will draw even more current.

You should check your GFCI rating too. Typically, these come in 15 Amp and 20 Amp versions.

My experience with what I THINK you are describing as a "portable GFI that plugs into the extension cord" is limited to a GFI headed extension cord (ie. it looks like an extension cord but the male plug in end is a GFI). As it is "portable" it gets banged around in moving, plugging and unplugging it...you might try to determine is the unit itself has been damaged and is not working properly.

If it were me, I'd seriously consider either rewiring a GFI breaker into the subpanel that provides that circuit or wiring in a "resident" GFI (as a plug in box in your GH)and running an extension cord from it, BUT as a rule, extension cords are not the best way to "extend" power, *especially* heaters!!

I once ran a small 110 heater from a properly rated extension cord but had the cord coiled around a reel. I left it running in a room to help dry some sheetrock and the reel of cord fused itself together.