Preheat your grill or grill pan over medium heat.
In a small bowl, mix together garlic powder, onion powder, paprika, salt, and pepper. Rub this spice mixture evenly over the chicken breasts.
Drizzle olive oil on the grill or grill pan. Place the chicken breasts on the grill and cook for about 6-7 minutes on each side, or until the internal temperature reaches 165°F. Remove from heat and let rest for 5 minutes.
While the chicken is resting, toast the slices of bread until golden brown.
Spread the mashed avocado evenly on two slices of the toasted bread. If using, spread mayonnaise on the other two slices.
Slice the grilled chicken and layer it on top of the avocado. Add a slice of cheese on top of the chicken, followed by fresh spinach and tomato slices.
Top with the remaining slices of bread, cheese side down.
Return the sandwiches to the grill or a skillet over medium heat. Grill for about 2-3 minutes on each side, or until the cheese is melted and the bread is golden brown.
Slice the sandwiches in half and serve warm.