If you use EDT cores (double E) as I do the best method I have found is either buy them gapped or gap the outer legs using spacers (card or paper) at 1/3rd the calculated gap, this also has the advantage of reducing the centre leg gap and hence the fringing flux that effects the winding but of course introduces fringing flux around the gapped outer legs.
I usually find a ty-wrap around the whole assembly is enough to hold the gaps closed. It's much easier to measure a thickness than a gap IMOP but you could use commonly available sparkplug feeler gauges on a plate of glass, however I must confess to never have tried grinding cores!
I must confess, the question doesn't make much sense to me. Either you have professional grinding tools, then measuring the material removal exactly shouldn't be a problem. Or you are using DIY methods, then the surface will be likely neither flat nor parallel and height measurements gives only a rough estimation of effective gap. In both cases, inductance measurement is a way to get the exact effective permeability rather fast.
yes i would be wary of low cost gapping tools..because if ferrite is not gapped with sensitivity, then the ferrite can get fractures in it which can break at some future time..........ferrite is very brittle, and yet rock hard to grind......the ferrite has to be held very tightly, and yet not so tightly that it fractures.