Isn't that the diverter that you have taken apart? If there is a "black washer" under the nut, then the stem has to be unscewed to "push" it out. But usually, a faucet stem that looks like yours does NOT have the gasket and unscrewing the unit removes the entire mechanism. I have never heard of a "Galtmaster" faucet, which means it is probably some other brand with the Galtmaster name on it. The handles like like Harcraft, but can also be generic.