There are sometimes when your assembly almost has to be in the GAC and when you need to debug it.  This was difficult since if you registered a .NET assembly into the GAC the debugger wouldn't break on it.  This is because the .PDB file information didn't get loaded since it's not in the same directory as the DLL.  Here's how to work around that.

1) Install the Assembly in the GAC
2) Start-Run, %SYSTEMROOT%\Assembly\GAC
3) Navigate into the folder with name of the DLL that you want to debug
4) Navigate into the folder with the version number and public key that you want
5) Copy the .PDB file into this directory

Now you'll be able to run a debugger against code running in the GAC.