Coincidentally, Mullvad, Windscribe and IVPN all worked when I was in China behind GFW, while more popular options did not.
Seems like there are VPNs, and then there are VPNs.
It sort of worked for me, but it was very unreliable. I tried Proton and Astrill, both of which worked much better.
Mullvad is pretty good overall though.
I'm a bit curious about how that works. I love Mullvad but routinely I find sites like Reddit completely block it. Even yesterday someone posted a Debian wiki link[0] and I was blocked. It's not all of them but Reddit is a big killer. So I thought China would block all of them (aren't they known?)
Fwiw I'm not switching from mullvad
[0] https://news.ycombinator.com/item?id=46252366