Cube the tofu and veggies into large pieces so that they dont fall off the skewers easily. Mix soy sauce, garlic, pepper flakes and salt in a bowl. Brush the mixture liberally on the tofu cubes and let chill to marinate for half an hour. Brush so that the pepper flakes are spread over all cubes. You can also sprinkle more gochugaru as needed.
Then Skewer the tofu cubes, melon and peppers onto skewers. Sprinkle more gochugaru/pepper and salt on the melons and peppers.
Grill on a grill skillet or grill, until tofu has nice grill marks. Turn the skewers around to have grill marks or on atleast 2 sides of the tofu. Serve as is or over salad, with sweet chili sauce.