Gluten free meals on CX do not have much sugar in the selection either. The desserts are usually freshfruit. For breakfast, you can opt not to have the yoghurt.
It is better to ask for the gluten free meal than to ask for a medical specific meal of no wheat and no sugar. Trust me, it will be very very bland!
I don't eat wheat either and I have had the gluten free meal and its quite ok even in Y. Its best out of HongKong as they have special gluten free bread which are really yummy!