I don't remember any rubber gasket, the rear window is just kind gooed on with butyl window seal muck.

As for the trim you are absolutely right - drill out the rivets and it pops off pretty easy. Remember to get the one on the outside too - there's three total. And be careful with that piece of trim, it's thin aluminum, don't pry much or you'll bend it. It too may have window seal goo on it so go around the edges carefullly prying it up after the rivets are out.

To put it back on, use some appropriate sized sheet metal screws or, as I did, tap the holes - there's just barely enough metal there to get a thread or two cut - and put in 1/4 20 screws. I even found some stainless steel screws with hex key heads, they almost look like the rivets that came out.

Oh, and BE VERY CAREFUL when drilling your car - you can go through interior trim if you're not careful. Go slowly.