Bring a large pot of salted water to a rolling boil. Add gemelli pasta and cook according to package instructions, usually about 8-10 minutes, until al dente. Drain the pasta and rinse briefly under cold water to stop the cooking process. Let it cool completely.

While the pasta is cooling, fill a medium saucepan with cold water and gently add the eggs. Bring the water to a boil over medium heat, then cover and turn off the heat. Let the eggs sit for about 9-12 minutes. Transfer the eggs to an ice bath for a few minutes, then peel, roughly chop, and set aside.

In a large mixing bowl, whisk together mayonnaise, Dijon mustard, coarse grain mustard, apple cider vinegar, and sugar until smooth and creamy.

Add sliced celery, shaved fennel, and finely diced red onion to the dressing. Fold in the cooled gemelli pasta, ensuring all ingredients are well-coated.

Carefully fold the chopped hard-boiled eggs into the mixture, season with kosher salt and freshly ground black pepper to taste, mixing until evenly integrated.

Transfer the salad to a serving dish, sprinkle with sliced scallions and paprika. Chill in the refrigerator for at least 30 minutes before serving.