To start, for the gas issue, try adding a pro/pre-biotic to the food, which will help aid digestion. Adding plain yogurt will sometimes help, but the higher amount of good bacteria that a pro/prebiotic provides would likely work better.
As far as adding more calories/fat to the food, you can try Evanger’s game meats. They are not a full diet, but are meat-only, so are to used as a topper not a replacement. That way you don’t have to worry about anything like extra minerals, you’re just adding fat and calories.
