Mix Gochujang paste, oil, vinegar, grated ginger, and grated garlic, soy sauce, and sugar in a medium bowl.
Marinate meat:
Remove the skin.
Slice the meat ¾ inch (2 cm) thick. Cut the slices into strips about as thick as your finger. Place in the bowl containing the marinade.
Combine well until the meat pieces are coated with the sauce.
Cover and refrigerate for 4 hours or up to 12.
Remove the meat from the fridge about 20 minutes before cooking if you have the time.
Cook:
Heat a large nonstick pan or cast-iron skillet on high heat. If the pan is not large enough, fry the belly pieces in two batches.
Place the meat in the pan and arrange the pieces in a single layer. Cook for 3-4 minutes, flip the pieces and reduce the heat to medium.
Cook for another 3-4 minutes or until cooked through. If the meat starts to get too dark, reduce the heat and move the pieces in the pan.
Remove with a slotted spoon leaving as much of the rendered fat behind.
Serve immediately.
Notes
Weighed with the skin still on, you will have a bit less once you remove the skin. You can keep the skin on if you wish, but it becomes somewhat chewy when fried.