The manifold gasket is a one-piece item and seals the exhaust as well. Replacing the gasket rather depends if it gets damaged when you remove the inlet. You could put a thin film of RTV sealant on the manifold before you replace it - just to be sure, but I suppose you should really replace the gasket. It's fairly common for the exhaust part of the gasket to blow (usually at the back), but not the inlet.
Les.