Although they're often referred to as the same thing, they're actually different. Tamari sauce comes from Japan and is made by pressing the liquid from fermented soybean paste (miso). It's naturally gluten-free, but some manufacturers add wheat so be sure to read the ingredient label to ensure that it's certified gluten-free. It's also slightly thicker and darker than gluten-free soy sauce. Gluten-free soy sauce is Chinese and has been fermented with rice instead of wheat (like many regular soy sauces).