TRY AT YOUR OWN RISK
I just got a call from a friend of mine asking how to copy a file from the GAC. I quickly told him and then asked around to see if it would be good for a blog post.
So here it goes, it is pretty straightforward.
Open up DOS Start->Run “cmd”
Change to the assembly folder CD C:\windows\assembly
It will probably look something like this.

Now CD into the GAC folder and then finally CD into the Namespace of your assembly. There will be a folder with the assemblies version and key, CD into that directory and copy out the dll.

That is all there is to it, if anyone knows more about the structure feel free to fill in the gaps.
—Brendon Schwartz