Gmvault can fail for multiple reasons, for example Gmail can cut the connection and throttle you for a while. There are also bugs left or GMail emails that cannot be downloaded (email corruption ?).

For this reason, there is a --restart option for the sync and restore mode (sync being the backup). The restart mode is a resume mode that will restart the backup or restore near from where it failed.

With that mode you should be able to download your 6 GB on disk.