With GVM 9 as far as I can remember you can track the process title of openvassd to know when the sync is finished. The feed sync consists of syncing the nasl files via rsync to your local machine and openvassd to load the meta data of the nasl files into the redis db.
With GVM 11 and therefore in 20.08 too the whole scanner architecture has changed. The daemon openvassd has been changed into an openvas application and an ospd-openvas daemon.