Turn the water off your toilet by adjusting the valve around its back.
Flush the toilet to remove as much of the water as possible from the toilet.
Fill the toilet with white vinegar. Fill past the gray ring stain.
Close the toilet lid and let it sit for one day to give the white vinegar time to loosen the stain.
Scrub the gray stains off the toilet with a toilet brush. Work until the stains are gone or until they are as light as possible.
Scrub the gray stains with a pumice stone if they are still visible.
Turn the water back on the toilet and flush it.