Barley is a gluten grain and should not be eaten by anyone who is completely avoiding gluten.
Kombu is a type of kelp or seaweed that can improve the digestibility of beans and legumes. It can be found in some grocery and most "health food" stores.
Bring the water to a simmer with Kombu and tamari. Add mushrooms and seaweed, and simmer about 10 minutes. Add tofu and turn off the heat. Mix a little broth to the miso to dissolve. Break up clumps and return it to the pot. Do not heat after the miso goes in. Different miso pastes have different flavors and saltiness. Taste the broth and determine if it tastes too thin, and add more dissolved miso if needed.