In a medium bowl, combine the tuna, mayonnaise, celery, red onion, pickles, Dijon mustard, garlic powder, salt, and pepper.
Mix until everything is well combined.
Butter one side of each slice of bread.
Place four slices butter-side down on a work surface.
Top each with a slice of cheddar cheese.
Divide the tuna mixture evenly over the cheese.
Add another slice of cheddar if desired for an extra cheesy sandwich.
Place the remaining bread slices on top with the buttered side facing up.
Heat a large skillet or griddle over medium heat.
Cook the sandwiches for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is golden brown and the cheese is melted.
Slice in half and serve immediately.