you can't repair a head gasket. there are some places that will sell you a chemical repair product, but I don't think it is worthwhile unless you are going to trade the car at once. I recommend that you have the head (heads if v6) removed, magnifluxed for cracks, checked for warp-age, and replaced with new gaskets. The cost will be more than just replacing the head gasket, but long run confidence in the repair will be worth the difference.
