Cut the pork tenderloin into one-inch cubes and place them in a medium mixing bowl.
Whisk together the gochujang, tamari, sesame oil, minced ginger, and minced garlic to create a smooth marinade.
Pour the marinade over the pork, tossing to coat evenly, and let it sit for at least fifteen minutes.
Chop the bell pepper and red onion into one-inch pieces suitable for skewering.
Thread the marinated pork, peppers, and onions onto skewers, alternating between meat and vegetables.
Preheat a grill pan or outdoor grill to medium-high heat and lightly grease with a touch of oil.
Grill the skewers for three to four minutes per side, or until the pork is cooked through and the edges are slightly charred.
Garnish with sliced green onions and sesame seeds before serving hot.