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.