It has been a long time since GFW blocked *.appspot.com and I've accepted the fact. But surprisingly a lot of people are using GAE with go-agent as proxy and it works for some area, I've tried that out myself as well, the result is it's quite unstable, for newly applied GAE application with go agent, this works sometimes, most times this proxy get blocked as well. but this seems not the case for others, why is that?
I had a theory for this partially blocked websites, .appspot.com cloud obviously had an address space of IPV4 , and the ip address is allocated dynamically to the machines in different way, GFW blocked machines based on IP addresses. so sometimes my server machine get a non-blocked IP address, so it works; most often it is allocated a blocked area, so it don't work. This is just wild guess.