GFCI 20 amp breaker pops....regular plug works
You don't need another GFCI, just put a regular receptacle here

The breaker's a GFCI already (between the TEST button, and 680.71 requiring GFCI protection for the existing tub), so you don't need another GFCI device to add to the confusion.  Simply put a regular 20A receptacle in with a "GFCI Protected" label on the faceplate, and you should be fine provided the GFCI breaker trips when you push its "TEST" button.