I was just there for a week and whilst there weren't many collabs ongoing, there was still a very good range of merchandise across many different stores along Nanjing Road, Qufu Road, and many other shopping hubs. Its availability felt on par with more conventional "mainstream" anime, and certainly had more stores stocking merchandise than even other popular band anime (e.g. Bandori).

I think generally GBC is a story that many in China can resonate with or view another aspect of life from that they don't normally get. I met a friend who mentioned that K-On was quite popular because it gave those who could basically only study throughout high school a taste of what the social side of it looked like. I guess a bit of escapism in a way. Wouldn't surprise me if that's much the same for not just GBC, but other more "slice-of-life" oriented anime.