For example if you want to copy System.Data.Linq.dll then you have to enter following,
cd WINDOWS\ASSEMBLY\GAC_MSIL\System.Data.Linq\
windows stores GAC assemblies at (windows\assembly\GAC_MSIL\
now type DIR to list all files/folders inside that folder.
Now you can see a folder(s) with version number and key.
(This is the place where actual dll file is saved inside GAC)