Thanks for answering. The YHM for the gas block fits the picatinny rail (on the upper) just fine. It does not fit the weaverrail on the gas block, because the screw does not fit within the grooves. I bought a 10/32 screw from the hardware store and ground the threads down to the shank to see if this would allow it to sit properly. Unfortunately the thinner screw fits the groove, but the threads at the tip prevent me from situating the sight flat on the block. If the gas block had a picatinny rail configuration, it would have worked without a hitch.
I have a GG&G on order, so I'll know shortly if it works.