Gaining

Military MRE's (Meals Ready to Eat) have a variety of dinners and they are highly caloric. and fun to cook. Google for them and some army surplus stores stock them as well.

I bought one to try before I got one to keep in my backpack. I would have to be desperate to eat them. Make sure you go slow at first to minimize possible digestive upset (either what was listed or the opposite...)

Walmart sells some in sporting goods.