Preheat oven to 400°F. Line two baking sheets with parchment paper.
Cut the chicken into 1/2-inch strips and add them to a medium bowl. Cut the peppers and onions into 1/2-inch strips then add them to a second medium bowl.
In a small bowl, whisk together the garlic powder, ground cumin, dried oregano, paprika, chili powder, 2 teaspoons kosher salt and 1/2 teaspoon pepper. Drizzle 2 tablespoons olive oil on the chicken then sprinkle with half the seasoning and toss to combine. Drizzle the remaining 2 tablespoons olive oil on the vegetables then sprinkle with the remaining seasoning and toss to combine.
Divide the chicken and vegetables among the sheet trays. Bake until the chicken is cooked through, 20 to 25 minutes.
Remove the baking sheet from the oven and serve the fajitas with tortillas and your choice of toppings, such as guacamole, salsa, cheese and sour cream.