Scratch my previous answer, turns out gateau is the legitimate spelling. However, gateaux is the plural, so that's probably why you saw that in the text. Dictionary source here.
OK final edit with irrefutable, straight-from-the-game-code answer! The game text I extracted from BW2 a while back has two files which are almost identical except one is plural and one is singular.
Extract from file A:
Extract from file B: