Some common issues if scans doesn't return any results:
- You don't have nmap installed or not available within your PATH
- The targets are not answering to an ICMP Echo Request
-> Check the "Alive Test" setting of your Target definition and try some of the other available methods
- SELinux is enabled and blocking the scanner from doing its job
- You're using a custom scan configuration which doesn't include the "nmap (NASL wrapper)" and "Ping Host" NVTs from the "Port scanners" family
Only points 2. and 4. of the above are valid for the Greenbone VM (GCE). All prerequisites like nmap are already available / installed and SELinux isn't installed / enabled.
The mentioned "Ping Host" plugin allows to configure a "Report about unrechable Hosts" to "yes" so you will get at least some notes if a remote host is considered as dead. In that case you will get no results.
Disclaimer: Answer of a NVT Dev @ Greenbone