I bought a 2012 Chevy 2500HD with a 6.0 gas. I have to tell you, it pulls very well. I even pulled my Takeuchi TB153 which weighs 12,500 on trailer that weighs 5K from 4500 feet to 6500 feet and I got 6 mpg doing it. I am still a diesel guy especially on the heavier loads but the these newer gas trucks with at least a 6 speed tranny are not incompetent at towing previous generations of gas trucks have been.
Finding the right diesel used would likely be ideal if your going to pull heavy and pull alot but if you want a new truck and gas allows you to get what you want and if your pulling under 15K combined with equipment and trailer, I would consider going gas.