Combine olive oil, crushed garlic, chopped rosemary, and chopped thyme in a covered container; let stand for 2 hours or longer.
Preheat an outdoor grill for medium-high heat and lightly oil the grate.
Put potatoes in a large saucepan and pour in enough salted water to cover; bring to a boil; cook over medium-high heat until mostly tender, 8 to 9 minutes. Drain potatoes and transfer to a bowl.
Remove garlic from the olive oil mixture and discard. Pour oil over potatoes, sprinkle with garlic salt and black pepper, and toss to coat.
Place potatoes skin-side down on the grill over direct heat; reserve remaining oil. Grill for 10 minutes; move potatoes to indirect heat until completely cooked, 5 to 10 minutes. Transfer to a serving bowl and drizzle remaining oil over potatoes.