Preheat your grill or grill pan to medium-high heat (about 400°F/200°C).
Pat the chicken breasts dry with paper towels. Brush both sides with olive oil, then season with garlic powder, smoked paprika, salt, and black pepper.
Place the chicken breasts on the grill. Cook for 4-5 minutes per side, or until the internal temperature reaches 165°F (74°C) and the chicken is fully cooked through.
While the chicken is grilling, lightly toast the sandwich buns on the grill or in a toaster for 1-2 minutes until golden.
Remove the chicken from the grill and let it rest for 2-3 minutes. Meanwhile, spread mayonnaise (if using) on the bottom half of each bun.
Assemble the sandwiches: Place a lettuce leaf on the bottom bun, followed by the grilled chicken breast, tomato slices, and red onion slices (if using). Top with the other half of the bun.
Serve immediately while warm. Enjoy your juicy grilled chicken sandwich!