My spec was 1.30mm±0.02mm, but the mold is measuring 1.28mm or 1.29mm, so a light shave would be 1.25mm, but I actually fully cut them off on mine and the caps still stay on.
Thing is, official MX spec that I found was 1.31 +/-0.2 mm. GMK molds are actually OG Cherry molds, so they should work fine on Cherry stem.
You are saying yours are 1.28 and they still somehow have problems. I find that to be peculiar because obviously stem thickness is just one of the factors, and shape, size and volume of the knob on the stem may actually have a lot to do with the problem, but that's just my speculation.