Gateau - gateaux - gâteau - gâteaux
The word gateau (the French for 'cake') is a singular. You can have one gateau. The plural (regular in French for words ending in -eau) is gateaux. So it is an error to write
gateaus or gateauxs. It is probably best to use the native word 'cake', but skilled pastry-makers, chefs and so on like to use French to lend an air of sophistication and prestige to their work.
The word can be written with a circumflex accent gâteau and gâteaux. This is better practice in French. It seems best, when using a French word, to use French orthography: but the circumflex is not used in native English words, and need not be used in words borrowed from other languages. (If you want to use one: in Microsoft Word, use the 'Insert' menu and the 'Symbols' to find an appropriate character, or press Ctrl + shift + ^, followed by the letter. Other programs have other methods.)
The pronunciation of both gateau and gateaux is 'gat-oh'. (The '-x-' is silent.) In French, there is no perceptible difference in stress between the two syllables IPA: /gæt o/; in British English, the first syllable is stressed ('GAT-oh', IPA: /ˈgæt əʊ/, but in American it is the second, 'gatOH' (IPA: /gæt ˈoʊ/).