GMail drive is a Shell extension (an experimental tool) that creates virtual filesytem based on GMail allowing you to store and retrieve files from GMail.
Once installed, you can find a drive named GMail drive in Windows Explorer. You can access the contents by login in. Once you try to open the drive, a login message pops up and after you have entered your GMail address and password, you are taken to the virtual drive built around your account.
You can also drag and drop the files into the GMail drive. When you create a new file in the GMail drive, an email is created and it appears in your email account and the attachment would be the file created by you.