Soak four wooden skewers in water for at least 20 minutes to prevent them from burning on the grill.
In a medium glass bowl, whisk together the gochujang, coconut aminos, toasted sesame oil, rice vinegar, grated fresh ginger, and minced garlic until smooth.
Cut the pork tenderloin into 1-inch cubes and the pork belly into slightly smaller 0.5-inch cubes to ensure even cooking and fat rendering.
Toss the pork cubes in the marinade, ensuring every piece is well-coated, and let it marinate in the refrigerator for 15 to 30 minutes.
While the pork marinates, chop the red bell pepper into 1-inch squares and cut the green onions into 1-inch lengths.
Thread the marinated pork tenderloin, pork belly, bell peppers, and green onions onto the skewers, alternating the ingredients to distribute the pork belly fat.
Preheat a grill or a cast-iron grill pan over medium-high heat and lightly brush with a high-smoke point oil if needed.
Grill the skewers for 3-4 minutes per side, turning occasionally, until the pork is cooked through and the edges are beautifully charred and caramelized.
Remove from the heat and let the skewers rest for 2 minutes before serving to allow the juices to redistribute.