Why are my backups not showing in the "Backup Restoration" interface of WHM?
The Backup Restoration interface in WHM only allows you to restore backups that are stored locally on the server or backups that are configured with an FTP remote destination.
Other remote backup types such as Google Drive etc, will not be accessible from the Backup Restoration interface in WHM.
You can use the Backup Restoration interface in WHM if you manually move the backups from the remote destination to the local backup destination.
cPanel Documentation - Backup Restoration