Place racks in upper and lower thirds of oven; preheat to 425°. Whisk sesame seeds, oil, gochujang, and soy sauce in a large bowl. Add squash and toss to coat. Divide squash between 2 rimmed baking sheets, arranging in a single layer. Roast, rotating sheets once, until tender and browned on some edges, 25–30 minutes. Serve topped with scallions and salt.