Pete Wehner, once Mike Gerson’s deputy in the White House, and a friend of colleague of Mike’s (and mine) since the early 1990s writes a fact-based response to Matt Scully’s piece attacking Gerson at National Review Online.
Wehner, someone singled out by Scully in his piece, as an honest guy writes:

In the World According to Gerson, Scully argues, the president’s speeches are the work of just one man, and his name is Michael Gerson. But certain facts belie the charge.
In a Sunday evening C-SPAN interview with Brian Lamb earlier this year, Lamb asked about how the September 14 National Cathedral speech was written. Gerson answered this way: “I worked on that with Matt Scully and John McConnell, who were writers that I’ve become very close with.”
When asked about the speechwriting process and State of the Union addresses, Mike said that when he would get back from Christmas break, he “would go into the season working with some fine writers that I’d hired — people like John McConnell and Matt Scully and others — to put together a draft.”

He goes on to highlight a half-dozen or so other instances where Mike singles out Scully or their speechwriting partner John McConnell for praise and concludes, “If people who know Mike Gerson don’t recognize him in the Atlantic Monthly profile, there is a good reason why.”
Finally, Timothy Noah has a smart roundup on the dustup in Slate here.
To the degree this is a fight between two men, all the points have been made. Matt has made his points in a highly public forum and Washingtonians will always look at Mike with a bit of a raised eyebrow. That is truly unfortunate because Mike is a beautiful and brilliant writer and a man with a tremendous heart. In days past this is the sort of thing that would have been handled by Matt saying to Mike, “Let’s take this outside” and two of them would have gone at it for a bit and it would have been over and that would have been that. I realize that lacks in sophistication but perhaps it would have been better than all of this.