Gm never used any exhaust gaskets under their cast iron manifolds..
I never put any on either after trying them once,and having one blow out not long afterwards..
I just put the manifolds on without any after a good scraping to get all the carbon and rust off both the head and the manifold--doesn't hurt to put a straight edge on the manifold and ensure it is flat too--some "bowing" in the center can be ignored,as the bolts will clamp it flat to the head as you tighten it..
I'd only use gaskets on headers,and the copper ones are the only ones worth using in my opinion..anything else burns up in no time..