In a medium bowl, combine the drained tuna, mayonnaise, Dijon mustard, celery, red onion, garlic powder, salt, and pepper. Mix until well combined.
Preheat a skillet over medium heat. Butter one side of each slice of bread.
Place two slices of bread, buttered side down, in the skillet. Top each with half of the tuna mixture and a slice of cheese.
Place the remaining slices of bread on top, buttered side up.
Cook for about 3-4 minutes, or until the bottom is golden brown. Carefully flip the sandwiches and cook for another 3-4 minutes until the other side is golden and the cheese is melted.
Remove from the skillet, slice in half, and serve warm.