The gasket is a rubber O-ring that lives in a groove under the outer threaded collar. Just be sure that the gasket is in place as you tighten the mount and you'll be fine.
+1 on the Greenlee punch for the hole.. those things are amazing. I used one to punch a hole into 16g stainless stock for a custom hood mount. it required one of my bigger wrenches, but the hole was clean. I do recommend a bit of 3 in 1 oil to help with the cutting.
I was on hamcity, and they have a Larsen NMO-K mount, thats what I'm looking for I think. I dont see anything about a gasket though.