GMail Drive is a Shell Namespace Extension that creates a virtual filesystem around your Google Gmail account, allowing you to use Gmail as a storage medium.
I know it's not a big deal and many ppl (imo) are using such tools, but there's still debate on its compliance with
Google Gmail Program Use Policy.
One of the stated No-nos:
"Modify, adapt, translate, or reverse engineer any portion of the Gmail Service;
Reformat or frame any portion of the web pages that are part of the Gmail Service."
Some real risks of using it (IMO):
1. Google may block the program altogether someday.
2. Google may disable your account.
3. When Google makes changes to Gmail, the program may break and trap your files inside your gmail account (at least temporarily).