I'm only a parent, so take what I have to say for what it is - limited to my own family's experience
I have a dd who has severe food allergies - brain fog is very real for her when exposed to certain foods. It's much easier to cope with now that she's a teen, but when she younger the spaciness was intellectually incapacitating (but thankfully temporary lol).
That said, I wouldn't wait on further eval in order to see what changes diet brings. If the assessment last year was only for gifted and only included ability testing, I'd suggest looking into a neuropsych eval, based on the gap in subtest scores and notes re reading and handwriting. FWIW, you'll probably have an answer re will the gluten-free diet help before you'll have a chance to get into a neuropsych for an appointment - there are often long waits to get in to see a neuropsych. In the meantime, track diet/behaviors and also watch schoolwork/etc closely - the neuropsych will be interested in both development history and how he's impacted now with classroom work and homework. I'd also suggest talking to him to see what he feels about reading/handwriting/etc - he's old enough that he may have very valuable insight into what's going on.
Re seeing an educational specialist vs a neuropscyh, this is just my opinion, but I'd see a neuropsych first. We chose to see an ed psych first for my 2nd 2e child, and while it was helpful (pinpointed the area of challenge, as well as gave us some wonderful recommendations re path forward), we were only able to get ability/achievement testing and eventually needed to seek out a neuropsych eval anyway to answer further questions to fully understand what the challenge is. The ed psych couldn't give us an official diagnosis, which would have been helpful in advocating. To be fair, our dd's challenge isn't as straightforward as our 2e ds' diagnosis, but in hindsight I wish we'd started with a neuropsych eval.
Best wishes,
polarbear
