From Seo Wiki - Search Engine Optimization and Programming Languages
It works by building a filesystem with the Filesystem in Userspace loadable kernel module, and manages communication with Gmail through a special purpose Python library called libgmail. The actual GmailFS is also implemented in Python.
The speed of this filesystem is limited by the speed of the user's Internet connection, and the upload/download speed of the Gmail server. Files can be any size, and are stored as (segmented) attachments.
The Official website news dated 13 December 2009, says "Unfortunately the GmailFS project has come to an end. libgmail has ceased being maintained by its developers, and as a result libgmail no longer works with the latest Gmail interface (and has not done so for many weeks). Without a working libgmail, GmailFS does not function, so the end of libgmail also spells the end of GmailFS."
Some users of GmailFS use filesystem-level encryption to protect the information they store with it.