Replace is preferred (by the way, head bolts are as well)...
But obviously people re-use head bolts, and people do reuse headgaskets. I wouldn't reuse unless you -absolutely- have to.. and if that is the case, clean it up VERY well, and use copper gasket spray on it.
But may as well not reuse them... replacements are cheap, and if you do it right once, you won't have to re-do it later
