Combine the mayonnaise, ketchup, and garlic in a small bowl. Cover and refrigerate for at least 30 minutes before serving.
Heat oil in a deep-fryer or large saucepan to 350 degrees F (175 degrees C).
Fry plantain chunks until golden but not browned, 5 to 7 minutes. Drain on a plate lined with paper towels. Allow the plantains to cool slightly, then smash each piece of plantain in a tostonero or with the bottom of a clean can. Fry the flattened plantains a second time until golden and crisp, 5 to 7 minutes more. Season with garlic salt.