I think you misunderstand the characteristic being measured by the GO portion of a ring gage. The GO ring represents the MAXIMUM allowable diameter which may pass - ergo every undersized shaft will pass through the GO portion, but that's only half the inspection. If the shaft passes through the GO portion, it must also be tested on the NO GO portion which represents the minimum diameter of the shaft specification. If it passes through, it then is a NO GO. (Think trying to drop a 1/4 inch shaft through a 1/2 inch GO/NO GO ring gage with +/- tolerance of 0.010 where the diameter of the "GO" is 0.510 and the NO GO is 0.490 - obviously it will pass though BOTH Go and NO GO.)
ASIDE:
Until I saw this post, I never realized the terminology could be confusing, but now I see it can be. Do not feel bad about being confused. Many of us old timers learned by on-the-job training and may very well have asked such a question, but we always had some older old timer to set us straight. In this context, NO GO means it should not go through. Both GO and NO GO must be tested to assure the part is in spec..