Ostensibly, the gasket is necessary to create a proper seal inside the air box; apparently, you're supposed to "semi-permanently" install the gasket inside the air box.
I have one of these K & N filters waiting to go into mine when the current disposable, paper filter is spent, and have been pondering how to get the gasket inside the air box without removing the whole air box assembly. I've tried test-fitting the air filter (without gasket) into the air box, and it seems to be a good fit already; I've pulled out a ruler, and the K & N filter appears to be the same dimensions as the paper filter I already have. So all of that, in turn, has led me to wondered whether the K & N gasket is really all that necessary...