2tablespoonslight soy sauce(or a gluten-free alternative)
Cut three shallow slits across the top of each chicken breast, to help the marinade better penetrate the meat.
In a small bowl, whisk together the olive oil, soy sauce, garlic powder, and black pepper. Divide the mixture into two small bowls. Use one of them and a pastry brush to brush the chicken breasts on both sides with the marinade.
Heat a George Foreman grill or a grill pan on medium-high heat and lightly grease the grill.
Grill the chicken breasts until cooked through and no longer pink, flipping them midway through cooking. For medium (8 oz) chicken breasts, this should take a total of 5 minutes on a dual-contact grill, or 5 minutes on each side in a grill pan. The best way to ensure that they are fully cooked is to use an instant-read thermometer. It should read 160-165 degrees F.
Transfer the chicken breasts to a sheet of foil and use a clean pastry brush to coat them with the second unused portion of the marinade. Cover them with another sheet of foil and let them rest for 5-10 minutes before slicing and serving.
If your chicken breasts are larger than 8 oz, they will need more time on the grill. Try adding 1 minute per side (on the George Foreman grill, flip them and cook for 2 more minutes), then check their temperature to see if they're fully cooked.