Yeah, I use a small screwdriver and press in on it and then wobble or pull it off. The part you are pushing in on realeases a clip which is in a groove just below where the splines are.
Is your gasket worn out and leaking? I never knew there was a gasket there. I would think it stops water from coming in the cab.
If it were me I'd get some form a gasket and shave off the excess.