Product review (5 stars): This is the extra think gasket kit for older Gerber toilets.
This is the super sized rubber gasket kit required to fix older Gerber toilets. Mine was  manufactured March 17 1992 (date stamped into the porcelain of the tank) and is a 28-290 model. There is also an A21 stamped into the tank but I'm not sure what that indicates.

The new gasket and tank screws worked perfectly.

Word of warning: The orange and blue home supply stores carry a Fluidmaster extra thick gasket which you may think will work. It will not, some older Gerber toilets need an even thicker, deeper, fuller gasket, than the Fluidmaster extra thick gaskets. If you want to try anyway, because you're adventurous like that, don't fill your tank all the way on your first test flush; Less water for you to mop off your floor. You're welcome.