I was having a problem with the tank leaking water to the toilet bowl. I have changed many flappers in other toilet brands.  When I first investigated I found that Koehler uses a gasket and it looked like it would be very difficult to change. Upon further investigation I found that it be much easier to replace than changing a flapper. I ordered a new gasket from the factory and received it with a few days.  It was a very simple repair.