Combine tuna, scallions, celery, capers, mayonnaise, mustard and ¼ cup shredded smoked gouda in a bowl. Stir gently but thoroughly to combine. Season to taste with salt and pepper.
Preheat the broiler. Butter both sides of the bread and arrange in a broiler pan or baking sheet. Broil, flipping once, until golden brown, 2–3 minutes per side.
Divide the tuna mixture evenly between the toasts, spreading it all the way to the edges. Sprinkle the remaining shredded gouda evenly over top and dust with a little smoked paprika. Broil until cheese is melted and bubbling, 3–5 minutes. Serve at once.