it's not the gasket that's skimmed. sometimes the cylinder head is machined to ensure it is as flat as possible to ensure a good seal between it and the cylinder block. we often call this machining process skimming.
Will go along with v64 paul. Often if a cylinder head gasket goes it can be through excessive heat which in turn could cause broblems with the cylinder head or block, possibly distortion. Skimming the head or block, will create a good seal when the new gasket is fitted, The gasket can not be skimmed.