In a small bowl, mix the garlic powder, za'atar, cumin, sumac, garam masala, salt and black pepper until combined. Rub the mixture all over the pork belly and then place on the prepared baking sheet. Place the pork belly in the oven and roast for 45 minutes. At 45 minutes, increase the temperature on the oven to 400 degrees F and continue roasting for an additional 30 minutes. Remove from heat and set aside to rest for 10 minutes. Once rested, slice the pork belly in 1/2 inch slices.