Cook the elbow macaroni in a large pot of salted boiling water according to package directions until al dente. Drain and rinse briefly with cool water.
While the pasta cooks, prepare the hard boiled eggs and dice the celery.
In a very large mixing bowl, combine the cooled pasta, drained tuna, diced eggs, and diced celery.
In a small bowl, whisk together the mayonnaise, Dijon mustard, and garlic powder until smooth.
Pour the dressing over the pasta mixture. Gently fold everything together until evenly coated.
Season generously with salt and pepper. Start with 1 tsp salt and ½ tsp pepper, then taste and adjust after chilling.
Cover the bowl tightly and refrigerate for at least 2 hours, or ideally overnight, to allow the flavors to meld.
Before serving, stir the salad, taste, and adjust seasoning if needed. Serve cold.