Combine wheat gluten, yeast, and garlic powder in large bowl. Stir in 1 cup broth and soy sauce until dough forms, adding more broth if necessary. Knead dough in bowl with spoon 3 minutes, or until elastic. Shape into 2 loaves.Place loaves in large saucepan, and add remaining 4 cups broth, onion, garlic, and enough water to cover seitan. Cover, and bring to a boil over medium heat. Reduce heat to medium-low, and simmer 30 to 45 minutes, or until seitan is firm. Remove from heat, and cool in broth.