Go/NoGo gages measure Pitch Diameter. They aren't really intended to measure the Major or Minor diameters. Pitch diameter is the area in the thread where the cross section of the thread, and the groove of the thread are equal. Most specs allow for 3 turns max with the NoGo, if the hole is deeper than 3x the diameter. With shallow holes, this usually wouldn't be allowed because it significantly compromises the strength of the thread. Also, the right gage needs to be used for the class of fit requred, 2b or 3b for internal, 2a or 3a for external. Class 2 fits are generally more common than class 3, which have tigher tolerances and better fit up than class 2.
Jason