Combine mayonnaise, ketchup, and garlic in a small bowl. Cover and refrigerate sauce for at least 30 minutes before serving.

Heat oil in a deep fryer or large saucepan to 350 degrees F (175 degrees C). Fry plantains in hot oil until golden but not browned, 5 to 7 minutes. Drain on a paper towel-lined plate. Allow plantains to cool slightly.

Smash each plantain in a tostonero or with the bottom of a clean can. Fry flattened plantains again in hot oil until golden and crisp, 5 to 7 minutes more. Season with garlic salt and serve with sauce.