In a small bowl, whisk together the tamari, rice vinegar, and gochujang until smooth to create the stir-fry sauce.
Cut the chicken breast into bite-sized cubes and season evenly with sea salt and black pepper.
Heat the sesame oil in a large skillet or wok over medium-high heat until shimmering.
Add the chicken to the skillet and cook for 5-6 minutes, stirring occasionally, until golden brown and cooked through; remove chicken from the pan and set aside.
In the same skillet, add the chopped bell peppers, sliced zucchini, minced garlic, and grated ginger, sautéing for 3-4 minutes until the vegetables are tender-crisp.
Return the cooked chicken to the skillet and pour the prepared sauce over the mixture, tossing well to coat all ingredients.
Stir in the sliced scallions and roasted peanuts, cooking for an additional minute until the sauce has thickened slightly and everything is heated through.
Serve the Kung Pao chicken immediately over the warm cooked brown rice.