Gaining

Meat. Sugar. Starch. There's plenty of options. Just eat lots of high-calorie, high-fat foods that you're allowed to eat, and watch the weight pile on

You might also try some dairy alternatives. While I'm not totally dairy-free, I've had to move away from cow's milk, and I've found almond milk to be a very viable alternative. It's great for baking (I've tried this with cakes, cookies, brownies...) I haven't tried making ice cream with it, though.

Happy baking!

Fall in love with mayonnaise. Japanese made a cuisine out of it, for once.

There are a lot of non dairy alternatives.
Very tasty is vanilla almond milk, there is cashew cream which is just like real cream, MANY ice creams including soy, almond, coconut....almond yogurt tastes funny, but soy yogurt is magical, child like memories of Trix yogurt come to mind. I love Silk's Very Very Vanilla soy milk, tastes like Ensure-ish. Lots of options. Plus they are all cholesterol free as only animal food products (meat, dairy) contain cholesterol.