If the builder used proseal, it will be hard... real hard. I can't imagine getting in there without pulling the wing (i.e. tank) off but that's just me. I haven't had my wings on yet; just relaying my experience during building when complying with the SB from Van's.
Some people use a gasket, some people don't. I've used a gasket on one, didn't use it on the other. I didn't find it made much difference in terms of removal.
If you decide to take the plate off (as opposed to say using some of the "wicking" solutions that have been posted at various sites over the yeas), be very careful about having a bunch of crud getting into the fuel tanks. I'm not sure how that's avoidable.
A heat gun might not be a bad idea to soften up the proseal, although, again, I've never done it with tanks that have had fuel in them so I can't speak to how safe this would be.
I would try to confirm if the problem is coming around the plate or from the screws. It'll be relatively easy to fix if it's the screws. Remove, clean, put a bunch of ProSeal on 'em, stick back on.