GO gauge - made to the minimum headspace
NO-GO gauge - made to beyond the max headspace dimension for a newly chambered gun
When they ream the chamber into the back of a barrel blank, the machinist needs to know how far to go into the back of the barrel. That is what the GO gauge is for. If the GO gauge does not go into the chamber, a cartridge wont fit because the chamber is too shallow.
A NO-GO should not fit into a newly chambered gun. If it does, it means the chamber is too long and the cartridge will not be supported by the chamber enough. If the headspace is too large, you can run into problems with ignition because the firing pin does not stick out far enough from the bolt face to light the primer reliably. Also if the chamber headspace is on the large side, it's not good for your brass (it gets blown out too much to fill the chamber) and it also leaves no margin for wear in the parts. The bolt lugs and barrel extension will wear over time and headspace will increase.
If someone is building this for you, they should check the headspace with your bolt. They don't need the whole carrier, just a stripped bolt. It's a liability/reliability/professionalism thing. The real problem is with a used or out of spec bolt or barrel extension.
B