How does this work? You don't need to disclose secret sauce per se, just enough to understand the legality and robustness of using it.
In terms of tech stack, GFWaaS is a mix of Rails, Apache Guacamole, and headless browser. If you just want to solve your own problem, you could simply setup Guacamole on a server in China, and use VNC in your browser.
The real challenge is dealing with the Chinese Internet. For example, to get reasonable connection speed for VNC, choosing the right cloud service provider is really the key. You have to figure out a way to get an account from the major providers in China (many of them, if not all, require you either live in China or have a local entity), and test the speed and stability of the connections. And, in some cases, because of the bad traffic peering across Chinese ISPs, you website may work relatively well with one ISP but not others, so you have to test with different ISPs.
GFWaaS streamlines the whole process so you can focus on testing your website, instead of finding the right cloud service provider and setting up software etc by your own.
Hope I have explained what you asked, and sorry for my English.