Place potatoes in a large saucepan and cover with water. Bring to boil. Reduce heat; cover and simmer for 4–5 minutes or until crisp-tender. Drain; pat dry with paper towels. Sprinkle with garlic salt and pepper.
Grill, covered, over medium heat for 10–12 minutes or until tender, turning once.
In a small bowl, combine the mayonnaise, yogurt and seasonings. Serve with sweet potatoes.