When you go to a restaurant in the United States, it is customary to pay extra money for the service of your waiter or waitress. This extra money is called the tip or gratuity.
Usually you, the customer, decide how much money you want to leave as a tip (it is customary to leave between 10% and 20%).
However, in some restaurants, the restaurant automatically adds an extra percentage to the bill, which is listed as “gratuity.” Restaurants often do this when there is a table with a lot of people. It is a way of automatically making sure that a tip is paid. When there is a charge for gratuity on your bill, then you don’t need to add an additional tip; the gratuity is the tip.