that sure looks like strip caulk, as in 3M strip caulk (that used to be gray, but now the gray cannot be found, although black is very available). There are other similar products that are gray or off white if you prefer those colors. It's intention is to keep out water, and to remain pliable, not to 'cement' in a part but whether GM calls it a 'cement' or not, I do not know..
. I do know that Chevrolet cars thru the 50's, 60's, and 70's used it in many places where one part is bolted or screwed to another. It's ONLY purpose I can see is to be 'pliable', and keep out water... I use a little almost anywhere I screw something onto a body.
