It would seem that the hole size for their Grip-Lock tuners is a closely-held secret. It doesn't show up on any of their pages. Seems like a pretty critical detail to leave out. Maybe they don't need holes! Maybe they're magic tuners! :laughing7:
But, the question has come up before so emails were exchanged with the threat of strong letters to follow, and they finally admitted that a 10mm hole would be just perfect, and that a typical Schaller or Sperzel cut was fine. I've installed them in Schaller holes without getting upset, so go with that.
Be aware that Warmoth cuts their tuner holes very exactly, and the Schaller hole is 9.92mm as opposed to the 10mm Hipshot is calling out. Also, while all neck woods are very hard, some are more brittle than others, so even though .08mm is just a gnat's whisker of a difference the holes might need to be opened up a smidge. A bit of 220-320 grit sandpaper around a pencil/dowel/something to finesse the hole a bit might be in order. Wouldn't want to split your headstock.