the plenum top does not have a gasket as such, it appears that it was sealed on from factory with Evostik or similar, basically looked like some kind of thin varnish coloured stuff to me, so i used evostik and it worked very well, minimal mess.
tbh i have refitted them before without any sealant becuase they are milled very flat and seal well considering the oil and gunk that ends up around everywhere, but belt and braces etc