A go no go gauge is just what it says it combines a pair of anvils that can be adjusted to just let the part between them. Next to this there is a pair of anvils that are just to narrow to let the same part through.
fig 5500 go no go gauge
The example above was made by the Conventry Gauge and Tool Company. These gauges were made to cover a whole range of sizes in 1/2inch increments. They are designed so they can be used quickly and accurately to check whether a part was within the tolerances required by unskilled workers.
The black round part is to make it easy to hold over a long period of time. The small aluminium disk is for numbering it so it is clear which part it is set up for. This often has the sizes it has been set to marked on it.
The anvils screw in and out and can then be locked in place. It might seem odd that all four can be adjusted. However if the outer pair are set using slip gauges to let the part through and then locked, it is necessary for the inner pair not only to be the right size but in the right place relative to the first pair.