Assembly installed into the GAC not showing up in Visual Studio
Visual Studio never lists all items in the GAC. It only lists items that are noted in a particular registry key.
Please see this knowledge base article for more info: How to display an assembly in the "Add Reference" dialog box
You can always add a reference to the assembly by browsing to it on disk.
I've created a tool which is completely free, that will help you to achieve your goal. Muse VSReferences will allow you to add a Global Assembly Cache reference to the project from Add GAC Reference menu item.