Combine olive oil, rosemary, thyme, and garlic in bowl; cover and set aside to allow flavors to infuse, at least 2 hours.

Preheat an outdoor grill for medium-high, indirect heat and lightly oil the grate.

Place potatoes into a large saucepan and cover with salted water; bring to a boil over medium-high heat. Cook until mostly tender, 8 to 9 minutes. Drain; transfer potatoes to a large bowl.

Remove and discard garlic from oil; pour infused oil over potatoes. Season with garlic salt and black pepper; toss to coat.

Place potatoes, skin-sides down, on the preheated grill over direct heat; reserve remaining oil. Grill for 10 minutes; move potatoes to indirect heat and continue grilling until fully cooked, 5 to 10 minutes.

Transfer grilled potatoes to a serving bowl; drizzle with reserved oil.