Place all the ingredients into a 9 by 9 inch casserole dish. Allow the chicken to marinade for 8 hours or overnight.
Preheat grill to medium high heat. Remove the chicken from the marinade, allowing the excess to drip off. Discard the remaining marinade.
Place chicken on the grill for 7-8 minutes. Flip over and cook an additional 7-8 minutes or until no pink remains and chicken reaches 165°F. Rest 3-5 minutes before slicing.
OPTIONAL GRAPEFRUIT SALAD DRESSING
If would like to make a chicken salad with the leftover grapefruit chicken, place all the ingredients into a jar, cover and shake well. Store extras in an airtight container in the fridge for up to 12 days.