1. Preheat grill to medium-high heat.
2. Combine garlic, chipotle, and 1 tablespoon oil; rub evenly over chicken. Sprinkle with 1/4 teaspoon salt and 1/4 teaspoon black pepper. Place on grill rack coated with cooking spray; grill 3 minutes on each side or until done. Remove from grill; keep warm.
3. Combine remaining oil, remaining salt, remaining black pepper, and chili powder in a large bowl. Add bell peppers and onion; toss gently to coat. Place vegetables on grill rack; grill 5 minutes on each side or until soft and charred. Place tortillas on grill rack coated with cooking spray; grill 30 seconds on each side or until lightly charred. Remove from grill; keep warm.
4. Thinly slice chicken. Divide chicken, bell peppers, onion, and lettuce among tortillas.