Preheat grill to medium-high heat.
In a small bowl, combine olive oil, salt, black pepper, and garlic powder.
Brush the chicken breasts with the olive oil mixture.
Grill the chicken for 6-8 minutes per side, or until cooked through.
Remove the chicken from the grill and let it rest for a few minutes.
Slice the chicken breasts into thin strips.
Toast the bread slices.
Spread mayonnaise on one side of each bread slice.
Layer the avocado slices, lettuce leaves, tomato slices, and grilled chicken on top of 2 bread slices.
Top with the remaining bread slices and cut the sandwiches in half.
Serve immediately.