Slott has stated he does the old "Marvel Way" style where he writes an outline and then the artist draws it out and Slott adds more to it. He did state it was slower then the method the guys above do cause of the back and forth, which is different from purely writing from a script and the artist goes from there.
And you'd be surprised how many Gage has cowritten, cause the latest story arc was the first time Gage has been on for the new volume.