1. Preheat grill to medium.
2. In a small bowl, mix together garlic, rosemary, and 1 teaspoon olive oil. Set aside.
3. Cut off a thin slice of each potato lengthwise to create a even base for potato to rest on. Slice off ends of each potato. Rest potato lengthwise between two chopsticks or wooden spoons to act as a stop for the knife and carefully cut vertical slits in each potato about every 1/8th inch.
4. Brush potatoes with remaining 1 tablespoon olive oil and season with pepper to taste. Place potatoes on grill, cover, and cook until almost tender (15-20 minutes).
5. Spread garlic and rosemary mixture over top of potatoes, cover, and continue to cook until soft in the center and crisp around the edges (15-20 minutes).