We present GSO-Simulcast, a new architecture designed for large-scale multi-party video-conferencing systems. GSO-Simulcast is currently deployed at full-scale in Alibaba's Dingtalk video conferencing that serves more than 500 million users. It marks a fundamental shift from today's Simulcast, where a media server locally decides how to switch and forward video streams based on a fragmented network view. Instead, GSO-Simulcast globally orchestrates the publishing, subscribing, as well as the resolution and bitrate of video streams for each participant using a centralized controller that is aware of all network constraints in a meeting. The controller automatically modifies stream configurations to meet the participants' real-time network changes and updates. In doing so, GSO-Simulcast achieves multiple goals: (1) reducing video and network mismatch, (2) less path congestion, and (3) automated stream policy management. With the deployment of GSO-Simulcast, we observed more than a 35% reduction in the average video stall, 50% reduction in the average voice stall, and 6% improvement in the average video framerate. We describe the principle, design, deployment, and lessons learned.