Preheat a grill to medium high. Put the potatoes in a medium saucepan; add the garlic, 1/2 teaspoon salt, and water to cover. Bring to a boil, then reduce the heat to medium low and simmer until the potatoes are tender, about 10 minutes; drain, discarding the garlic.
Meanwhile, mix the olive oil, rosemary, thyme, lemon zest, 1 teaspoon salt, and pepper to taste in a small bowl. Toss the zucchini in a bowl with 1 tablespoon of the herb oil. Rub the steak with a tablespoon of the remaining herb oil.
Grill the steak, turning once, about 10 minutes for medium rare (a thermometer inserted into the center of the steak should register 130 degrees F to 135 degrees F). Transfer to a plate to rest. Meanwhile, grill the zucchini, turning once, until well marked and crisp-tender, about 10 minutes. Transfer to a plate and sprinkle with half of the lemon juice.
Toss the potatoes with the remaining herb oil and lemon juice, the parsley, and salt and pepper to taste. Serve with the steak and zucchini.