Hello,
Please note that Google Groups is reserved for conceptual discussions that don’t involve debugging. I understand you may know this already but just ran out of options to get additional assistance other than Stack Overflow. In order for you to get proper assistance, I would recommend you to check out Google Cloud Platform Support. If you happen to be on a free trial, you may be eligible for free support (click on free trial).
Also, please take a look at this working example of gae/go and postgresql which also uses pgx as it may be helpful. This is part of the Cloud SQL quick start so should work almost out of the box (I say almost because you still need to add your own cloud sql instance information).