Please install the latest GCE release 4.3.14. There was a bug fixed most recently in GOS:
Bugfix: In some cases the default quality of detection for all vulnerabilities was set to 75% due to the OpenVAS manager not correctly generating its internal metadata database (#108670).
which might be responsible for this seen behavior reported here.
Additionally even with the newest version the following still applies:
For some reason my fresh GCE 4.3.14 installation has shown a “feed not reachable” message during the first setup causing a 0/0 count of NVTs as well. A subsequent trigger of the feed update via Maintenance → Feed → Update in the GOS menu has started a full sync again.