Answer:
The Supreme Court of Nebraska ruled that the core of the contract involved the sale of goods, since the Hospital was purchasing the one-ply roofing system. Even though labor was included, the contract was predominantly about a good, not a service. E.g. when you purchase a car, someone sells it to you, but the core of the contract is the sale of the car.
In other words, Gates was responsible for selling a roofing system (good) and he is responsible for any defects.