I often use shortcuts and forget that this is the directory/path name to the Global Assembly Cache (where BizTalk .NET components are “GAC-ed”).
It’s NOT this one: C:\windows\assembly\GAC_MSIL
See prior blog: What happened to the GAC in .NET 4.0
NOTE: This is not the “BizTalk GAC”, it’s just the .NET 4.0 and above GAC. I just added that to help searches find this blog. We use the GAC a lot in Biztalk. All BizTalk Artifacts, and any C# components (DLLs) that BizTalk artifacts call must be deployed the GAC. Thus they need a strong key filename set in the .NET project properties.