As you are well aware, all BizTalk programs must reside in the GAC. Many of use the “shell extension” to drag and drop programs to the GAC. But BEWARE, this tool is gone in .NET 4.0.
See here for the Microsoft Official Statement: https://msdn.microsoft.com/en-us/library/vstudio/34149zk3%28v=vs.100%29.aspx
Former link was moved by MSDN: http://msdn.microsoft.com/en-us/library/34149zk3.aspx
“Beginning with the .NET Framework version 4, the Assembly Cache Viewer (Shfusion.dll) is obsolete and has been removed. Use Gacutil.exe (Global Assembly Cache Tool) to view and manipulate the global assembly cache.”
For a good explanation, see the answer to this popular question on Stack Overflow (GAC).
Disk paths to the old and new GAC: