Brush 1 tablespoon olive oil over the red potatoes. Pierce potatoes with a fork and arrange onto a baking sheet. Spread baby eggplant pieces, red bell pepper slices, green bell pepper pieces, red onion, and garlic onto a separate baking sheet; drizzle 1 tablespoon olive oil over the vegetables. Stir 1/2 cup olive oil, garlic salt, oregano, basil, and black pepper together in a bowl.
Roast potatoes in preheated oven for 20 minutes. Put baking sheet into the oven and continue roasting potatoes and the vegetables until the potatoes are tender, about 25 minutes more. Let everything cool for 10 minutes.
Cut potatoes into bite-size chunks and put into a bowl. Drizzle olive oil and seasoning mixture over the potatoes and stir to coat; add the roasted vegetables and stir.