When people quit their nicotine habit, they tend to gain weight because of decreased metabolism. So by extrapolation, maybe it would be harder to lose fat if you quit, if you are already fat. Regarding gains, be more specific as to what you are trying to "gain". Muscle? Strength? Dick length?
Regarding strength, there was a university study several years ago that examined strength in wrestlers who chewed versus who didn't. Chewers had significantly less strength gains than the non-chewers.