A gateau is a type of cake. The word gateau is French for cake. However, in general usage, we consider gateau as a light sponge cake with a several layers with rich fillings. Therefore, the gateau is a more elaborate and fancy dessert than cake.
The gateau also takes a long time and more effort to make. Another important fact about gateau is that gateaux that are made fruits tend to wilt rather easily; therefore, they are usually intended to be eaten on the same day they are made.