In a small bowl, combine garlic salt, red pepper flakes, black pepper, and onion powder. Set aside.
In a large bowl, combine Worcestershire, soy sauce, and teriyaki marinade, reserving 1 1/2 cup of marinade for later. In a large bowl, place chicken thighs and pour 3 cups of Triple-Threat Marinade over chicken and marinate for 30 minutes.
Remove chicken from marinade, discarding marinade, and season on both sides with dry seasoning.
Preheat lightly greased grill to 350°F (medium setting).
Place chicken, skin-side down, on grill using indirect heat and grill, with grill lid closed, for 40 minutes, checking every 15 minutes. Be careful not to overcook.
Turn chicken and grill for an additional 20 minutes with skin side up.
Throughout the grilling process, baste frequently with reserved Triple-Threat Marinade, using a spray bottle.
Chicken is done when internal temperature reaches 165°F, or until juices run clear.