Rick is probably right, but you could always try to fix it first. It's a lot cheaper that way if you are just doing it yourself.
Take a grinder to the rust, and get every last bit of it out of there. Wipe it clean with a towel or rag, then immidiately primer it. If the entire area in question will be hidden by the spoiler, I wouldnt worry about paint - just douse it with primer. Tape off anything you dont want primered, though.
I'm not sure what to suggest for a gasket. However, I would suggest checking it again in a few weeks to see if the rust is coming back. If so, new hatch.