0 error [?] while importing from site library

Hi there,

can you make sure that there are no plugin/server caches or optimizations ( such as Redis ) running. Also check with your host that the gpsites.co domain can be reached.

I am confused about "check with your host that the gpsites.co domain can be reached".

By the way on my backend varnish runs with apache and Nginx. it is a sandwich method where the request goes Nginx->varnish->apache and responds to the request is in the reverse way.

why am I able to install other plugins and themes

The Site Library will try to import content to your site - Server side or plugin caches can disrupt the import process. You need to test without caches enabled to see if that is the issue.