Gâteaux is the French word for “cakes”; the singular is “gâteau.” Both are pronounced the same: “gah-toe.”
In French, gâteaux refers to any kind of cake, from cheesecake to sponge to fruit cakes to pound cakes. It can even mean pancake at times, or something savoury. It is not used for pastries, pies, or tarts (large or small.)
In English, gâteaux is used to mean a specific kind of cake: an elaborate, highly-decorated, rich, sweet cake, with layers, frosting, filling, etc.
Generally, the cake will also involve fresh fruit and fresh whipped cream, and generally doesn’t have a long life span.
The batter may have finely ground nut meal in it.
The word “gâteaux” was borrowed from French in the mid-1800s, and began acquiring its fancy meaning in English around then. Makes sense — if you’d made a very fancy cake, you didn’t just want to call it a “cake.”