Want to Take a DLL backup from GAC

Option-1:
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open command prompt and navigate to %windir%/assembly/gac_msil/ and use copy command to take a copy of the require dll.

Option-2:
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Want to do it in a windows way ,
Simply enter the following:

subst g: %windir%\assembly\gac

Now open Explorer, go to drive G: and you're all set.
Thanks to Jeff key

Option-3:
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Start->Run->type c:\windows\assembly\gac
There you are take a back up of the required dll and go ahead