être du gâteau

to be very easy

Something that is du gâteau is dead easy, the way English calls an easy task a piece of cake. Often heard in the negative: ce n'est pas du gâteau, it's no picnic.

Ne t'inquiète pas pour l'examen, c'est du gâteau.

Don't worry about the exam, it's a piece of cake.

In French, sweets have long stood for easy, pleasant things, the older expression 'c'est du nanan' (18th century) used candy the same way. The positive form 'c'est du gâteau' was recorded by the Académie française in 1952, and the negative form follows naturally: if it isn't cake, it's hard work.