GMS and GM engine sealant are two different things. they quit making GMS many years ago, it was a dark gray. it was replaced with GM assembly adhesive #12346141 which is light gray like the engine sealant just easier to take back apart and you should be able to get it still. people still call it GMS because they use it the same but there are some new parts guys that have no idea what your talking about.
ben, GM part number 88861417 is what you want.