Rub one side of each slice of bread with garlic. Place 4 bread slices garlic side down; top each bread slice with one teaspoon mustard and 2 slices Gouda.
Place remaining bread slices, garlic side up, on sandwich bottoms. Combine butter, cayenne pepper and black pepper in a small bowl; brush mixture over each side of sandwich.
Cook sandwiches in an oven proof skillet over medium-high heat about 2 minutes on each side or until golden. Place skillet in oven and bake at 400 degrees for 5 minutes or until cheese is melted. Slice sandwiches diagonally. Makes 4 sandwiches.