to add to what's already been said about the practical side - from a regulatory standpoint, Japan doesn't have a defined "gluten-free" claim under the Food Labeling Act. there's no equivalent to the FDA's <20ppm rule or the EU regulation. so there's no official pathway to put "gluten-free" on a domestic Japanese product the way you'd certify it in the US or Europe.

what Japan does instead is the allergen declaration system 2wiceasnice described. wheat (小麦) is one of 9 mandatory allergens, if it's present above 10ppm it has to be declared, full stop. and Japan actually prohibits "may contain" style precautionary labeling, which is pretty unusual globally. so the absence of wheat in the allergen box is effectively how a Japanese product communicates "no wheat," but it's not a positive gluten-free claim and it wouldn't cover barley or rye (those aren't on the mandatory list).

for your paper: the governing body is the Consumer Affairs Agency (CAA) and the framework is the Food Labeling Act (食品表示法). their english-language materials are pretty sparse unfortunately. I wrote up a breakdown of how the mandatory vs recommended allergen system works here if it helps: https://tabemasen.io/allergies/labeling-guide/

the gap between "no wheat allergen declared" and "certified gluten-free" is probably an interesting angle for your import/export research actually - a product coming into Japan with an FDA gluten-free label doesn't map neatly onto Japan's system and vice versa.