I havent tried the heat gun, that sounds lije a good idea.
I have done the permatex stuff though. It is for liquid gasket, like whats on the cases, it takes it off pretty well. But for stuck ass old paper gaskets it doesnt do anything. I used almost a whole bottle on a stuck base gasket without any effect.
The only method i have found to work is very careful removal of as much as you can with a razor blade. Then when its down to just the sticky leftover mess chuck one of these
http://solutions.3m.com/wps/portal/3M/en_US/Marine/Home/Products/Catalog/?PC_7_RJH9U5230GE3E02LECIE20S4K7000000_nid=GSGKXWRKYVbeRJ5GMFSMV0gl
into the die grinder and carefully let it do its job. If you try to go too fast youll mess up your sealing surface.
I use a yellow one.