Cut the pork tenderloin into 1-inch cubes and the pork belly into smaller 0.5-inch pieces.
In a medium bowl, whisk together the gochujang, tamari, rice vinegar, honey, sesame oil, minced garlic, and grated ginger.
Add the pork pieces to the bowl and toss thoroughly to coat, then let marinate for at least 15 minutes.
Thread the pork onto skewers, alternating between the lean tenderloin and the fatty pork belly pieces.
Preheat a grill pan or outdoor grill to medium-high heat and lightly grease.
Grill the skewers for 3-4 minutes per side, brushing with any remaining marinade, until the pork is cooked through and the edges are crispy.
Slice the cucumber into rounds and thinly slice the green onions.
Serve the skewers hot, garnished with sesame seeds and green onions, with the fresh cucumber on the side.