See the article on Bill Lahti's blog: Drag-Drop for an Android GridView.
For the Eclipse project source code, click the download link in the Attachments section below. It has been updated so it compiles cleanly with Android 4.4.2.
(Note: If you are viewing this in Chrome, click the download icon below to download. If you browse to the zip file first, you will find yourself in a Google doc viewer that requires installation of a download app.)