But there IS the back of an outlet from an adjacent room, that we were thinking we could tap into and connect to a GFCI blank (no outlets, just the GFCI test/reset button), then run the line out to the motor. (We'll expand the current outlet into a 2-gang box and then the GFCI reset will be there in that other room, no biggie.)

But the GFCI blank is rated for 20A.

The outlets in that line are rarely ever used, and certainly would never be used at the same time as the tub.

I'm guessing the reason we can't share the existing 20A line is that the motor is already using most of the 20A, and the heater needs 12A.

We're trying to avoid having to run a new 20A line!

Is it OK to use the higher rated (20A) GFCI breaker on a 15A line?
We're not neophytes with wiring. We rewired most of our house already!

15A, 20A, rewired/relocated/new wired many of our lights, did all the wiring for new additions, etc. My husband used to work do construction work in college, so he does pretty much everything himself. Regular runs, 2-way switches, 3-ways, the works.

He insisted a 20A GFCI would be fine on a 15A circuit, but I just wanted to check, get some other opinions.

We're DIY pros already.

This heater is the final step on our master bath remodel (completed gutted and rebuild from scratch). We didn't think about the heater ahead of time though, or else he would have run a new wire. Now, it's just a bit of a pain. Everything's newly tiled over and finished. (and since I do all the mudwork, there is no way I'm letting him cut more holes in the sheetrock, which he tends to do at the drop of a hat! unless there's just no other options.)