The GAC can be found at
C:\windows\assembly. There is a custom shell extension enabled that prevents you seeing the GAC dlls as you would normally see the file system in Windows Explorer. There are several ways to see the dlls in the GAC.
Copy the DLLs to Another Location
Open the command prompt and type the following:
xcopy . C:\GacDump /s /y
Disable the Shell Extension in the Registry
Open the registry editor and add/set the HKLM\Software\Microsoft\Fusion\DisableCacheViewer DWORD value to 1.
Use the SUBST Command
Run the following command (assuming you don't already have G:) mapped.
> SUBST G: C:\WINDOWS\ASSEMBLY