Heat the oil in a large skillet or wok over medium-high heat.
Add the chicken and sprinkle it with the garlic powder, salt, and pepper. Cook the chicken, stirring constantly, until the chicken is cooked through, about 8 minutes.
Transfer the chicken to a plate and cover it with aluminum foil to keep it warm.
Add the onion and carrot to the skillet. Sprinkle it with a pinch of salt. Cook, stirring constantly, until you can almost see through the onion, about 5 minutes.
Add the zucchini, yellow squash, and the red pepper, if you are using it. Sprinkle the vegetables with another pinch of salt. Cook the vegetables, stirring constantly, until the squash is cooked through but not soft, about 5 more minutes.
Add the chicken back to the skillet and stir it into the vegetables. Add the lemon juice and soy sauce, stirring to combine. Cook for 2 minutes. Season to taste with salt and pepper.
Serve the chicken and vegetables with cooked brown rice.