8mm-06 and 8mm magnum are the wrong size, so don't bother getting them. There are 8mm gages out there, it's just that they are hard to find. The main stores don't carry them anymore. You need 8mm Mauser. And, when you find them, you should buy all three: GO, No-Go, and Field.
Go is the smallest length. If the bolt on a rifle or the MG-42 bolt closes on a Go gage, then the head spacing is OK. If the bolt will NOT close, then the headspacing is TOO TIGHT and the firearm is NOT safe to shoot. You MUST be able to fully lock the bolt when a cartridge is in the chamber.
No-Go is intermediate length. If the bolt closes fully without interference on a rifle with a No-Go gage, then the headspacing is too large and the rifle MAY be dangerous to shoot, and probably if you do, the casing of the round will be overexpanded and useless for reloading. My MG-42 does close on a No-Go Gage and works fine, and I think that the tolerances are loose, and that the MG-42 is probably safe to shoot.
Field gage is the longest gage, and the one used to tell you that the headspacing is so large that the firearm (rifle or machinegun) is NOT SAFE UNDER ANY CONDITIONS to shoot. If the bolt closes on this gage, the gun should not be used! If the headspace is large enough for the bolt to lock down on this gage, then the headspacing is so large that you will probably always split or break the shell case, which will allow escape of high temperature, high pressure gasses out of the rear of the gun, and probably into your face.
Gas escape in smaller amounts, will erode and pit the face of the bolt. On old guns, you may notice a ring of pits corresponding to the diameter of the primer pocket on a cartridge. This will show you what happened when a bit of gas escapes from the rear of the cartridge around the primer. And it can happen without blowing the primer out the back of the cartridge. Over time, leaky ammo primer pockets cause this ring of erosion pits, especially on old military guns that have seen a lot of use and have used military ammo. I have seen this pit pattern of several of my MG-42 bolts and some k98 rifles. If headspacing is too large, the gas escape will be huge because of a ruptured case, and the shooter is in danger of serious injury.
Keep looking for 8mm Mauser gages. All collectors of old Mausers and machineguns SHOULD have them, and when you buy a new (old) gun, you should check them for headspacing BEFORE shooting them the first time. If you can't get the gages, had a local gunsmith check the headspace for you. They almost always have the correct gages, because any gunsmith worth his salt would have sooner or later worked on an 8mm Mauser.