Cook the pasta. Bring a large pot of well-salted water to a rolling boil. Add the gluten-free elbow macaroni and cook for 1 minute LESS than the package directs for al dente. Gluten-free pasta keeps cooking from residual heat and firms up as it cools.
Rinse and cool. Drain the pasta and rinse under cold running water for 30 seconds, tossing with your hands or a spoon to wash away surface starch. Toss with 1 teaspoon olive oil and set aside to cool completely.
Prep the vegetables. While the pasta cooks, finely chop the red bell pepper, celery, red onion, gherkins, and hard-boiled eggs. Aim for pieces about the same size as the cooked pasta for the best texture.
Make the dressing. In a large mixing bowl, whisk together the mayonnaise, yellow mustard, white vinegar, sugar, kosher salt, black pepper, garlic powder, and onion powder until completely smooth. The dressing should taste slightly too tangy and slightly too salty — the pasta will dilute it.
Combine. Add the cooled pasta, chopped vegetables, gherkins, and chopped eggs to the bowl with the dressing. Gently fold everything together until evenly coated.
Chill. Cover and refrigerate for at least 1 hour before serving. For best flavor, chill 8–12 hours.
Serve. Before serving, give the salad a good stir and check seasoning. Add a pinch of salt or splash of vinegar if needed (cold dulls flavor). Serve chilled.