Not every partition (or filesystem, or network share, or...) is local, meaning located on your computer ;) There are a number of protocols that allow you to access remote files as if they were on a local filesystem. Nautilus (the file manager) supports connecting to remote locations via SSH, FTP, WebDAV, NFS and others.
It looks like the mail client in question created such a reference. If I had to guess, it probably points to the cloud storage space connected with your Google account (Google Drive). You can mount this network share and then browse the files there as if they were on your local machine - or you can leave it alone, at your discretion.
EDIT: How you can remove this reference depends on how exactly the mail client created it. It may be a connection to your central online accounts.
You can find those in the GNOME settings ('Settings' in the menu at the top right or gnome-control-center). There should be a section 'Online Accounts'. If in this section there is an entry for 'Google', you can click on that entry and select what this account should be used for (mail, calendar, contacts etc.). If 'Files' is activated in this list, Nautilus shows a reference like you described. You can deactive the checkbox for 'Files', or just delete the Google account from your online accounts alltogether.
This is probably the simplest way to remove the reference - if the mail client used this way to integrate Google. If this way doesn't work, some detective work may be in order to find out what exactly the mail client did and how to reverse it.
