Preheat grill to high heat.
Drain juice from pineapple slices into a small mixing bowl.
Whisk in barbecue sauce, minced garlic, and soy sauce; set aside.
Place 6 pieces of foil (approx. 12"x18") on a clean working surface and spray with nonstick cooking spray.
Place a chicken breast in the center of each piece of foil.
Divide pineapple slices, peppers and green onions evenly on top of each chicken breast.
Season with salt and pepper, to taste.
Reserve 1/2 cup barbecue pineapple sauce; set aside.
Drizzle remaining barbecue sauce evenly over foil packs.
Fold the ends of the foil together and seal open ends closed tightly.
Place foil packs on the grill and close lid.
Cook for 15-20 minutes, turning over halfway through, until chicken is cooked through and peppers are tender.
Carefully open foil packs and serve with reserved barbecue sauce, if desired.