In a small mixing bowl, whisk together the gochujang paste, tamari, toasted sesame oil, maple syrup, minced garlic, and grated ginger to create a smooth marinade.
Cut the pork tenderloin and pork belly into 1-inch cubes and place them in a large bowl; season with sea salt and black pepper.
Pour half of the gochujang marinade over the pork and toss thoroughly to coat each piece.
Prepare the vegetables by cutting the red bell pepper and red onion into 1-inch chunks.
Thread the marinated pork tenderloin, pork belly, bell pepper, and onion onto skewers, alternating the ingredients for even cooking.
Heat a grill pan or outdoor grill to medium-high heat and lightly brush with oil to prevent sticking.
Grill the skewers for 3-4 minutes per side, or until the pork is cooked through and the vegetables show a light char.
During the final minute of grilling, brush the remaining gochujang glaze over the skewers for a glossy, flavorful finish.
Transfer the skewers to a serving plate and garnish with toasted sesame seeds before serving.