I've found the GMC tailgate panels in the pick and pull junkyards, but nothing from the new parts places.
Also verify whether your GMC logo should be in the middle of the tailgate or to the left side. If you have a stock tailgate it will have the correct placement of the logo. I believe they put the GMC in the middle of the tailgate up through about 77, so I think your 79 would have it on the left. From what I recall, Chevrolet's have it on the right.
My '75 had GMC centered on the tailgate metal in raised letters, and the aluminum panel used the same placement. The way the trim panels attach to the tailgate also changed a little over the years, so if you get a junkyard part you need to make sure you get all the parts.
Bruce