Soak four wooden skewers in water for at least 20 minutes to prevent burning during the grilling process.
Cut the pork tenderloin and pork belly into uniform 1-inch cubes and place them in a medium mixing bowl.
In a small jar or bowl, whisk together the gochujang, tamari, toasted sesame oil, rice vinegar, grated fresh ginger, and minced garlic.
Pour the marinade over the pork cubes, tossing thoroughly to ensure every piece is well-coated, and let it marinate for 15 minutes.
Chop the red bell pepper and zucchini into 1-inch pieces and season them lightly with sea salt and black pepper.
Thread the marinated pork and vegetables onto the skewers, alternating between the lean tenderloin, the rich pork belly, and the vegetables.
Preheat a grill or a cast-iron grill pan over medium-high heat and lightly grease the surface with a high-smoke-point oil.
Grill the skewers for 3 to 4 minutes per side, or until the pork is cooked through and the gochujang glaze has developed a deep, charred crust.
Remove the skewers from the heat and garnish with a sprinkle of sesame seeds and freshly sliced green onions before serving.