I am a type 1 diabetic and have a fair amount of success with mass gain, however mass gain was a result of my training, not the overall goal. In terms of training, I keep my workouts geared towards strength. For me strength workouts drive my blood sugars up more than they cause them to drop. Not sure what training plan your following now. But make sure you know your blood sugar responds to it.
As far as nutrition goes, I make sure I get at least a gram of protein per 1lb of body weight, the rest of my calories come primarily from fats like almond butter. I find I have been able to put on a resonable amount of muscle without gain any belly fat. Carbs I get from veggies. I always eat so that I am full, but not stuffed.
This is nothing crazy of scientific, but for me eating paleo, with the bulk of my calories from protein and fat work for me. Carbs will help with mass gain, but i think you may end up with more fat than you intended.
What is your height, weight, and body-fat? Also what are your A1C's looking like?