Gmail isn't a volume. What are you trying to accomplish? If you want quick access, you could use POP for it. Though I find it just as quick as much easier simply to forward a copy to my Mail.app.
What he is properly talking about is using gmail as a network drive as you can do in XP with Gmail Drive (i think thats what it is called) As far as I know thats not possible.
Browni beat me to it. I was going to use the GMailFS example from Linux though. (Where you mount gmail as a logical mount.) I do not know of anything to do this on the Mac.
I use the script at the bottom of this thread to imitate gmail drive. If you are accustomed to leaving Mail.app open, simply edit the script to not quit Mail.app upon completing, and the entire process is completely transparent. All the instructions are included: