It's an exclusive that you won't be able to get through Diamond, so if you're going to the show, grab it while you can!
If you won't be at NYCC, don't fret--the good news is that the full trade of G-Man Vol. 3 is currently scheduled for an April 2013 release.
To keep up to date on all things G-man, head over to www.chrisgcomics.com.
Also, Chris will kill me for this, as he hates drawing attention to himself but here's a nice interview he did with the American Library association in June of this year.