Soy sauce is a major condiment in cooking here, of course, and you might be surprised to learn that even though its primary ingredients are soybean and salt, it also includes roasted grain. That grain is often wheat.
So if you're looking for a gluten-free alternative for your cooking, here's one to consider: The company イチビキ (ichibiki) puts out a product called 小麦を使わない丸大豆しょうゆ (komugi wo tsukawanai marudaizu shouyu, "wheat-free whole soybean soy sauce") that is advertised as being as tasty as the real thing.
You can find this brand in some supermarkets. It runs about 300 yen for a 500 ml bottle. You can also order it online from retailers like ケンコーコム (kenko.com).