Gaining

Sounds like that might be a bit of a sweetness overload to me. But adding a bit here and there should give you a nice calorie bump. You should also try adding Heavy cream to your shakes as a way to boost the calorie load.

What Scarlet said, I like to mix heavy cream into pre-made chocolate milk. Makes it very rich and tasty.

Also, do you think it's from the richness of the heavy cream that causes you to have stomach issues? Or do you have some dairy sensitivity?

If you have sensitivity I'd suggest taking lactase before trying to drink heavy cream again.

(Nods) in that case I'd suggest building up to bigger amounts of heavy cream. I know a lot of new gainers who try too much too fast with heavy cream and wind up making themselves sick.