I ran into the same problem not long ago when i installed my nismo gt500 titanium one. I thought you put an allen key around the bottom or use another tool however you dont need any tools.
All you have to do is grab it with two hands and a cloth and twist with everything you got. If its the original knob and has never been changed, it is held on by some tough glue which you will find dries up and will be in circular threaded shaped pieces when the knob comes out.
If you get tired, take a break and try again. Hopefully this helps.