The Gerber tank model number 28-580 has three different variants, a 0.8GPF, 1.28GPF, and 1.6GPF, or Gallons Per Flush.

You can most commonly find the gallons per flush where the tank meets the bowl behind the toilet seat.

Select the gallon per flush your tank model 28-580 uses below

0.8GPF       1.28GPF       1.6GPF