Preheat oven to 400°. Place potatoes in a large microwave-safe bowl; microwave, covered, on high 5 minutes. Stir in chickpeas and onion; toss with 2 tablespoons oil, garam masala and 1/4 teaspoon salt.

Spread into a 15x10x1-in. pan. Roast until potatoes are tender, about 15 minutes. Cool slightly.

Place garlic and remaining 1 Tbsp. oil in a small microwave-safe bowl; microwave on high until garlic is lightly browned, 1 to 1-1/2 minutes. Stir in yogurt, lemon juice, cumin and remaining 1/4 tsp. salt.

Toss potato mixture with arugula. Spoon into pitas; top with sauce and cilantro.