SNI often allows you to discern the domain being accessed, and depending on how well you know that domain, the nature of the traffic flows can unwrap a lot more

SNI often allows you to discern the domain being accessed, and depending on how well you know that domain, the nature of the traffic flows can unwrap a lot more than most people would really like (timing and sizes of requests/responses - even wrapped with https, the size of a GET vs POST uploading data are pretty obvious). The GF doesn't care about 100% accuracy either. They won't always perfectly ban stuff. But they will hinder it often enough to make some "banned" service effectively useless inside China.