Gaining

Two biggies in processed food can be salt ( blood pressure impact) and sugar ( sugar highs and lows, messing with insulin levels and sensitivity, etc).

Reducing salt and sugar will probably help on their own. But some people (I'm one) don't digest large amounts of fat well, so that is another possible factor.

Mind you, I also have digestive issues with a bunch of healthy foods, so individual bodies will vary. Basically I'd say making individual changes and holding them for a couple of weeks can give you an idea of the specifics of what works for you.

Eating tons of junk food is definitely fun, but it works different for everyone and can lead to lack of energy, especially when combined with a more passive lifestyle.

You can eat clean and still consume tons of calories. Maybe you just need a feeder / chef to help you out