GSP uses some clever AJAX functionality from the AJAX Control Toolkit. All versions of GSP (which, as of this writing, is at version 2.4) ship with version 1.0.10920.0 of AjaxControlToolkit.dll. This is the newest version that can run under .NET 2.0.
For various reasons you might want to upgrade to a newer version of the toolkit. Because of versioning issues, there are some common pitfalls that can snag you. So I put together a little KB article to help explain the process and keep you out of the weeds (or help pull you out if you get sucked in). Click the link below.