my favorite food rule is eat things that look like what they are.
that eliminates alot of processed foods and chemical preservatives and sugary drinks.
i think keeping in mind the placebo effect and that few people only change a diet when dealing with behavior issues.
Parenting a kid with ADHD and especially a gifted one is complicated and is about the long haul.
keeping a too stringent diet may sap off parental energy that will be needed later for other things. also to broad a diet change may miss something simple that is a specific trigger. (some red dyes in drinks and red icing were a trigger for us for instance).
but go with what works for now. keep and open mind about what might work better in the future is what worked for us most of the time.