Soy sauce is gluten free if you choose some of the products made by Kikkomon or SanJ. Click on the brand names listed in the last sentence. Single serving packets of gluten free soy sauce (great for traveling) are available from The Gluten Free Pantry.
Many soy sauce ingredient lists contain the controversial ingredient caramel color. Click here for information about this.
A homemade substitute for soy sauce can be made from molasses and balsamic vinegar. Click here for instructions.