Cut the pork tenderloin and pork belly into uniform 1-inch cubes.
In a small bowl, whisk together the gochujang, tamari, sesame oil, rice vinegar, minced garlic, and grated ginger to create the marinade.
Place the pork cubes in a bowl and toss with the marinade until thoroughly coated; let marinate for at least 15 minutes.
Thread the pork onto skewers, alternating between the lean tenderloin and the richer 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, or until the pork is cooked through and the edges are slightly charred and caramelized.
Thinly slice the cucumber and arrange on a plate with a sprinkle of sea salt.
Place the skewers over the cucumber and garnish with sliced green onions and sesame seeds before serving.