My father-in-law will say "gate 'ooks" instead of "gateaux". He claims this is a regional/dialectal thing and that it was common in the part of Sussex where he grew up.
Is that likely, or is it more plausible that it was just him and a few of his friends mocking the spelling? My googling can find nothing useful on the subject.
I believe that 'gateaux' is the original French - so if using the word, the only real pronunciation would be 'Gatt-oh' in the same style as the native use. 'Gat-ooks' seems like some sort of niche corruption of the original, which is probably only valid within his certain circle rather than the wider (correct) use of 'Gateaux' (presumably without accent - throwing in a French accent on a single word in an otherwise English sentence has always seemed like an unusual idea to me).
There is no official pronunciation of local dialect that matches what you report. The local terminology for that is 'taking the piss'.
It's simply old people trying to be clever. All the rest of the jokes they could make are either racist or vulgar or both.
