Hi, I am trying to install a new 'Korky toilet fill valve and premium flapper' in my 81yr old dad's toilet.. I have never done this before, I shut off the water under the tank and removed the nut under the old fill valve, however when I attempt to remove the old fill valve only part of it comes out. I am left with a grey plastic tube with a black rubber gasket. When I attempt to remove it, I seem unable to because it is stuck any ideas? Thank you for any help.

Are you sure that you remove the nut holding the tube in place and not just the nut connecting the water supply tube. When the nut holding the tube of the valve in place is removed the tube should easily pull out.
Can you post a picture of the bottom of the tank where you removed the nut.