I've done a few gliding flights. They were all towed to height by a powered aircraft and then released. Winch launch is an alternative to towing, but sounded like a fairly terrifying alternative where you were yoinked into the air by a huge winch. The terror isn't an issue if the glider is pilotless. But IIRC they used something like a 7 litre engine to winch an incredibly light glider. Presumably it would require something really powerful to launch a cargo glider.