Me: Tell me, do you think that a single grain of wheat is a heap?
Mathhead200: Certainly not.
Me: And do you agree that adding a single grain could never turn a non-heap into a heap?
Mathhead200: Kind of. I think there is a middle ground between 'heap' and an emphatic grade of 'non-heap', which we shall call a 'pittance' of grains. And I do declare that adding a single grain will never turn a pittance into a heap.
Me: Ok. Now, is a single grain anything other than a pittance?
Mathhead200: Certainly not.
Me: And do you agree that adding a single grain could never turn a pittance into a non-pittance?
Mathhead200: No. You see, "pittance" turns out to mean a number of grains which could never be called a heap. So the smallest non-pittance is simply the smallest number of grains which could, conceivably, be called a heap. Don't ask me what that number is, but I don't see that I need to know. This is where I just am not what you are calling a 'relativist-gradualist'. Perhaps we should need an empirical survey of some kind, in order to locate the number in question. But the point is that it's there to be located.
Me: On the other hand, do we need to conduct an opinion poll (or whatever kind of survey you had in mind) to establish whether a single grain could ever be called a heap?
Mathhead200: No.
Me: And do you agree that adding a single grain could never turn a quantity not in need of a such a survey to establish pittance-ness into a quantity requiring such a survey?