Whisk oil, soy sauce, light corn syrup, lemon juice, sesame seeds, garlic powder, and garlic salt together in a medium bowl until well combined. Add chicken; stir to coat. Cover the bowl with plastic wrap; marinate in the refrigerator at least 2 hours.

Preheat an outdoor grill for medium heat and lightly oil the grate.

Remove chicken from marinade and shake off excess; thread chicken onto skewers alternating with onions, mushrooms, and bell pepper. Place on a plate and set aside.

Pour marinade into a small saucepan; bring to a boil. Cook until slightly thickened, 5 to 10 minutes.

Cook skewers on the preheated grill, flipping frequently, until chicken is no longer pink and juices run clear, 15 to 20 minutes. Baste frequently with boiled marinade during the last 10 minutes. An instant-read thermometer inserted into centers of chicken pieces should read at least 165 degrees F (74 degrees C).