(NOTE: This answer only applies to Windows; the process may vary and/or may not be possible on other OSs.)
I have my Google Drive folder mapped as my "G:" drive, which makes navigating to and referencing my synced Drive files and folders a cinch compared to having to go through my user folder.
As Al E. mentioned in his answer, you will need to download Google Drive for your PC to sync to/from a folder. Once you have that all set up, the
subst command should do the trick as far as mapping a local folder such as your Google Drive folder as a drive. However, just using
subst in the Command Prompt is only a temporary measure, as all "subst'd" drives are unmounted upon reboot.
To keep your subst'd drives between sessions, you should use Visual Subst, which is a simple, free, and lightweight yet highly nifty application that makes mounting local folders as drives really easy AND automatically remounts them at the start of a new session (if you so choose).