GCE VM backups do not require the installation of a connector. If your cloud credentials are using JSON key, please be aware that organizational policy may enforce an expiration date on these keys. To address this, refer to the following link to repl...

Ideally host should not be in not TRUSTED state, to over come this refer https://cloud.google.com/backup-disaster-recovery/docs/configuration/manage-hosts-and-their-applications#trust

No need to delete the application, Following steps will help to restore trust In the management console, click the Manage drop-down menu and select Hosts. The Hosts page appears listing known hosts, including their certificate status. Right-click the...

For GCE VM backups, it is not required to install a connector. However, for connector-based backups, the user needs to install the connector on the host. While adding the host in the management server, the user needs to provide the certificate.

Support Matrix https://cloud.google.com/backup-disaster-recovery/docs/supportmatrix-backupdr