G: is for GAC
From Jeff Key's .net blog
Using Explorer to get to physical files in the GAC
The Explorer extension the framework installs to view the GAC in %windir%\assembly doesn't let you do much with those files. If you try to access the physical folders below the assembly folder, you are redirected to the assembly folder. If you want to get at those files (and if you do, be careful), simply subst a drive to the %windir%\assembly\gac folder.
For example, if I want my GAC files to be represented by drive letter G, I simply enter the following:
subst g: %windir%\assembly\gac
Now open Explorer, go to drive G: and you're all set.
This is such a good trick I've added it as batch file with a link in my startup directory (where has autoexec.bat gone btw?). Using this trick it seems you can easily add assemblies from the GAC using the VS.NET add references dialog. The great thing about assemblies in the GAC is they're guaranteed not to move. You can even flip to compiling against the Rotor (and Mono?) GAC by remapping G: to the relevant directory. Okay, where's the catch? I get the impression you aren't supposed to do this.