Combine gluten-free soy sauce, apple cider vinegar, pineapple juice, brown sugar and garlic powder in a medium saucepan. Cook over medium heat for 20 minutes until sauce has thickened slightly.
Place chicken thighs in a bowl. Add 1/3 cup soy sauce mixture, ginger, and red pepper flakes. Mix together and set aside.
Slice zucchini into 1/4 in thick pieces. Remove stem and seeds from bell pepper, and leave in 2 large pieces. Remove stems from mushrooms, and clean off any excess dirt. Cut onion into thick slices, do not separate the rings.
Preheat grill or grill pan to medium heat (you can also just use a regular sauce pan). Toss the vegetables in olive oil and season with salt and pepper. Cook until vegetables are tender and grill marks appear. Remove from grill or pan and set aside.
Place chicken on grill and cook for about 5 minutes on each side, until the chicken is cooked to 165°F and there is no pink.
To serve, take the bell pepper and onions, and cut into bite-sized pieces. Quarter the mushrooms and zucchini. Toss the vegetables in 2 tbsp of the soy sauce mixture above.
Serve chicken and veggies with rice (pictured) or grain of your choice.
Notes
The teriyaki sauce can be made ahead of time and stored in an air tight container in the fridge for 2 weeks!