Preheat the grill on high heat.
While the grill preheats, rinse and pat-dry the chicken breast. Cut the chicken in half. Drizzle olive oil on top of the chicken breast and sprinkle with the garlic & herb seasoning.
Reduce the heat to low, then clean the grill. Spray with cooking spray and then place the chicken on the grill. Grill for 15 minutes, turning once. Grill until internal temperature reaches 165 degrees Fahrenheit.
While the chicken cooks, slice the tomatoes, avocados, and lettuce. Place on a tray. During the last minute of cooking place the hamburger buns on the grill in indirect heat.
Remove the burger buns from the grill, place the chicken breast on each bun. Top with 1 tablespoon ranch, a slice of tomato, lettuce, and avocado. Repeat, until all the chicken sandwiches are done. Enjoy immediately.