Gmail filesystem (gmailfs) is actually a type of a filesystem (just like NTFS and ext3). From what I've been able to tell, Google Filesystem seems to be an application which uses FUSE to mount a "drive" which, when written to, simply sends an email with an attachment to Gmail and, when read from, simply retrieves the email and its attachment. In other words it seems to be a filesystem comprized of emails as files or rather inodes. In other words, unless I'm missing something, gmailfs is only a part of what Google Filesystem application does.