Not sure which case cover gasket you mean...
There is a glued seal that goes around back of the hard top underneath it - it does close the gap to a degree.
10mm to me seems a bit too much, but you can read the posts on this forum that this gap is substantial. I initially had similar gap. But:
- I made sure that the plastic cones on rear fitting points go in all the way into the openings in the chrome pillar cover so that the rubber washer rests against the chrome
- I assembled the rear latches wrongly, looking at the wrong picture. I fitted the nuts under the metal plate through which the handle pivot goes. Maybe you made the same mistake.
Otherwise just check if it settled evrywhere as it should.