Prepare the charcoal grill for indirect grilling. Light the charcoal and let it ash over until glowing red.
Spread the lit charcoal evenly on one side of the grill, leaving the other empty. Place a drip pan filled with water on the empty side of the grill.
Pat dry the pork belly with a paper towel and place it on a large plate or tray.
Season with salt and pepper. If using a spice mix that contains salt, omit the salt.
Sprinkle the rub all over the pork belly slices until fully covered. Let sit for at least 30 minutes so the spices penetrate the meat. You can let it rest overnight or for 24 hours if time permits.
Place the pork belly slices on the grill grates over the heated side of the grill and let it grill for 1-2 minutes per side for the initial sear. If a flare-up occurs immediately, move it over to the charcoal grill's empty side (indirect heat side). Close the grill lid.
Maintain the grill temperature between 325-375°F (163-190°C) by adjusting the vents or adding more charcoal to ensure a consistent cooking temperature.
Grill the pork belly slices with the lid closed for 10-15 minutes. Flip the pieces and continue grilling for another 10-15 minutes or until the internal temperature reaches 145°F (63°C) for medium doneness.
Optional: If desired, you can baste the pork belly slices with the garlic chili or BBQ sauce during the last few minutes of grilling for added flavor and caramelization.
Once cooked, remove the pork belly from the grill and let it rest for a few minutes before slicing.