Jim Newkirk has released an alpha version of a utility to convert NUnit tests to VSTS tests. What he doesn't mention in the post is that he's using GAT to integrate this conversion functionality into VS05. Basically this conversion is an "unbounded recipe" which means that any time you right click on an item in the solution explorer, the NUnit Converter uses Visual Studio's CodeModel functionality to analyze the contents of the file. If the file is a C# file and has any NUnit test fixtures in it, NUnit Converter adds a "Convert NUnit Test Code" item to the context menu.
From a cursory glance at the code (which Jim was kind enough to send me) it doesn't look like it took very much GAT code to integrate into VS. Of all the files in the solution, there are only three that relate to GAT - the Conversion Action (i.e. the code that initiates the NUnit -> VSTS conversion when you select the context menu item), the Conversion Recipe Reference (i.e. the code that determines if the conversion menu item should be added to the context menu) and the Selected Project Item Provider (i.e. the code that retrieves the selected file from the Solution Explorer). There's also the XML file that defines the recipes. Everything else as far as I can tell handles the conversion itself and has nothing to do with GAT.
It's cool to see a real-world usage of GAT and that using GAT is a pretty low-impact effort given the VS integration benefits it provides.