In a covered large saucepan, cook potatoes in boiling water for 5 minutes; drain. In a large bowl, combine hot potatoes, olive oil, garlic, salt, and pepper. Toss well to combine. Using a slotted spoon, transfer potato mixture to a grill basket or grill pan.
For a charcoal grill, place grill basket or grill pan on the grill rack directly over medium coals. Grill uncovered about 25 minutes or until potatoes are golden brown and tender, turning grill basket or tossing potatoes in grill pan several times during grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place grill basket or grill pan on grill rack over heat. Cover and grill as above.)
Transfer potatoes to a serving dish. Sprinkle with green onions. Season to taste with additional salt and pepper.