If it is rubber gasket that is in good condition (tight fit, pliable, etc.) then you do not need sealer on it (you may want to use sealer to "glue" the gasket to the valve cover so it stays on the valve cover when you remove it later). I have used Permatex Ultra Copper for years and like it a lot - use the least amount possible (a little goes a long way) to prevent excess squeeze out.
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