No, you can't reuse a headgasket that has a copper fire ring. Not sure what you mean about sealer for the water jackets? Something to put on head bolts that pass through the water jackets? ARP thread sealant, Permatex Blue, or black RTV should work out fine.
I think he is talking about the head gaskets that are all copper. And yes I think you can re-use them a few times, but not like 20 times. But I am not 100% on that, you should contact the gasket manufacturer.
If you are talking about putting sealer anywhere on/around the head gasket, that is a big no-no.