yes, factor30, the written language is only a means of recording the spoken language, so written words that don't reflect spoken words are always at risk. In theory there's a word called hiccough, but in practice everyone writes hiccup now because that's how it's spoken.
For words that already end in S, it's hard to know how to spell them, or pronounce them. When we speak, do we say St Thomas hospital or St Thomases hospital?
I think the X on the end of gateaux, bureaux etc is pronounced as S (or actually Z, but the same principle). so saying gateauzes would be a bit of a mouthful. So gateaux' is probably right.