Due to circumstances involving a MPF package and Text Templating (t4), I had to add an assembly to the GAC after compiling if I wanted to be able to run the generator in the VS Experimental. So, I added the following line to the AfterBuild target of the csproj file:

<Exec Command="gacutil /i $(TargetPath)"/>

Unfortunately, I received the following error when I compiled: 

Error 155 The command "gacutil /i [PathToDll]" exited with code 9009. C:\Projects\AperioVisualStudioAddins\AperioVisualStudioAddins\AperioVisualStudioPackage.csproj 202 2 [DllName]

The problem is that when Visual Studio is building the project, it does not have access to the path. The way to make it work is to specify the fully qualified path and filename to gacutil.

<Exec Command="C:\WINDOWS\Microsoft.NET\Framework\v1.1.4322\gacutil.exe /i $(TargetPath)"/>