Bring a large pot of water to a boil with a ½ teaspoon of salt. Cook pasta in salted water until al dente. Drain well.
In a large bowl mix together drained pasta, onion, cucumber, grape tomatoes, pepperoni, mozzarella cheese, and black olives. Mix in garlic powder, Italian seasoning, and Italian dressing. Cover and refrigerate for 5 hours for the flavors to blend.