Kraft posted a detailed description and video of his project on the Hack a Day website. "Of course we can't make a real working gravity gun," explained Kraft in the video, "but we can make something that looked like it was working."
Using magnets from a StellaNova levitating globe to modify an already purchased gun, Kraft was able to create the illusion of a working gravity gun. The gun can't be shifted around without losing its cube, but a stable field does hold the cube completely.
You can read about the complete Portal process over at Hack a Day.