Preheat the oven to 400°F. Lay a clean dish towel out on the counter and spread the drained chickpeas on it to help them dry thoroughly.
Toss the chickpeas with the oil and arrange in a single layer on a baking sheet. Toss the cauliflower with oil and spices and spread on a baking sheet. Roast chickpeas and cauliflower, tossing every 10 minutes, until the chickpeas are crispy and golden and the cauliflower is golden and soft, about 35 minutes.
While the chickpeas are hot, toss them with the garam masala and kosher salt. Serve chickpeas and cauliflower warm or at room temperature in two separate bowls. Keep uncovered to retain crispiness.