The last update I heard is that the GFW does active probing to detect Tor bridges and relays. That is, there are computers inside China that are triggered by something, which then actually speak the Tor protocol to an IP to check if it's capable of subverting the firewall. If so, it gets blocked.

There isn't really any way around that which scales, which is why Tor is basically a non-entity in China right now. Everyone I know from the west who spends time in China just uses commercial VPN services, not Tor.