If you have cast iron manifolds,GM did not use a gasket between the manifold and the head.If you want to put one on you'll have to use some sort of aftermarket header gasket.Those cast iron manifolds were notorious for warping.Many times after they have been on the car and removed you will find they won't fit back on correctly.I have seen this many times.I hope this is the gasket you are refering to.
Yea, you really need to use one. The factory matched the head to the manifold, but over time it does warp. You can get a standard gasket at any auto parts store. I like the stock ones because they are easier to install. You run the bolts in just a little and they drop in from the top. The ears are basically U shaped instead of a hole like a header gasket.