Soak wooden skewers in water for at least 20 minutes to prevent burning during the grilling process.
In a small bowl, whisk together the gochujang, tamari, toasted sesame oil, minced ginger, minced garlic, and rice vinegar until the sauce is smooth.
Slice the pork tenderloin and pork belly into uniform 1-inch cubes and season them evenly with the black pepper.
Place the pork cubes in a bowl and toss with half of the prepared gochujang glaze, allowing the meat to marinate for 15 minutes.
Thread the marinated pork cubes and 1-inch scallion pieces onto the skewers, alternating between the lean tenderloin and the rich pork belly.
Heat a grill pan or outdoor grill over medium-high heat and brush with the avocado oil to prevent sticking.
Grill the skewers for 3 to 4 minutes per side, turning occasionally until the pork is cooked through and develops a beautiful caramelized crust.
Remove the skewers from the heat, brush with the remaining glaze, and garnish with sesame seeds before serving immediately.