In a large mixing bowl, whisk together low-sodium teriyaki sauce, pure pineapple juice, gluten-free soy sauce, dark brown sugar, minced garlic, freshly grated ginger, and toasted sesame oil. Add the boneless, skinless chicken to the marinade, ensuring every piece is well-coated. Cover the bowl with plastic wrap and refrigerate for at least 2 hours, or ideally overnight.

Preheat your grill or grill pan to medium-high heat (around 375°F to 400°F). Ensure the grill grates are clean and lightly oiled.

Remove the chicken from the marinade, letting any excess drip off. Place the chicken on the preheated grill and cook for 6-7 minutes on each side, or until the internal temperature reaches at least 165°F.

Add fresh pineapple rings to the grill and cook for about 2-3 minutes on each side, or until they become lightly caramelized and golden.

In a small saucepan, pour in the reserved marinade from the chicken. Bring to a boil, then reduce to a simmer for 5-6 minutes until thickened.

To serve, place a bed of fluffy white rice on each plate, top with the grilled chicken and caramelized pineapple rings, drizzle the thickened marinade, and sprinkle with green onions and sesame seeds.