GPF stands for Gallon per flush, generally the toilet's gallons per flush are printed on the porcelain behind the toilet seat. New toilets, per federal law, are only allowed to deliver 1.6gpf or less. In many years past, toilets flushed as much as 7-8gpf.
At first, manufacturers tweaked the valves and floats in the tank to reduce the water used without making any changes to the tank or bowl. The two most common adaptations were to install a flush-valve flapper which closes before all the water escapes the tank (early-close flapper) or to install a plastic bucket, or toilet dam, which retains some water in the toilet tank behind the dam, thus lowering the volume of flush.