GT Racers has three difficulties: Easy, Medium and Hard. There is also a car name editor in which you can rename any unlocked car.
Quick Race: A single race where you can do one to ten laps in any unlocked car on any track.
Championship: Go through all ten tracks in a chosen car. There are three championships: Bronze, Silver and Gold.
Completing Bronze unlocks five racing decals for each car (normal colours are no longer selectable), except for the Imperiale car which still has normal colours. Completing Silver unlocks the OAC Takemusu car, which has maximum acceleration and handling. Completing Gold unlocks the Imperiale car, which has maximum speed and handling.
Source: Wikipedia, "GT Racers", available under the CC-BY-SA License.