There is a good bit more work that goes into slotting the gauge than just a few minutes cutting the slot. As mentioned previously, the gauge has very tight tolerances. So after the slotting the slots have to be very carefully cleaned and deburred. Any small error in this could mess up headspace measurements or the diameters. So while it is certainly possible to have a machine shop mill a slot in it, you may not get the results you want. And I would not recommend this.
As far as slotting other brand gauges, it is a question of utility. If the gauge is oversized in diameter so that it only is useful for checking headspace or trim length, what will you gain by seeing the slot? You can’t check the profile of the cartridge against the gauge because the profile of the gauge is oversized.
Also, please keep in mind that our slotted gauge is patented, with the following utility patents: US9513097B1 and US9863748B2.