Donavan

Couple of things to check
1. Make sure the voltage wires from your house connected to the GFCI are tight and none are loose,
2. Also make sure when you installed the plug end the wires are tight.
3. Where you connect to the outlet must be getting wet for some reason, even with the protector.
4. Please understand GFCI do go bad you just might have a weak GFCI

How long have you had the Intex motor.

Can you run the extension to another outlet to see if the motor or circuit breaker trips after 15 minutes