GPF, or gallons per flush, is the standard measurement of water required to flush a toilet in the United States. The current federal standard for GPF is 1.6 gallons but recent advancements in toilet technology allow some toilets to flush as little as 1.27 gallons for a full flush.
Toilets which come with dual flushing capabilities (a flush for #1 and a flush for #2) often use less than 1 gallon per flush on the #1 side.