Gaining

Not everyone can gain weight, just like not everyone can lose weight. If you can't gain weight, a 140 pound gain is probably unrealistic. If you want to gain, you'll need to find time to eat.

Is it possible to add drinks to your day? So if you're consuming fizzy drinks/fruit juice/milk etc it'll all add some calories

Always be drinking a coke or pepsi... also steady flow of bite size candy is an easy way to keep your count up.

Consume 7k to 10k calories a day ( heavy cream works well since it's 50 cals per tbsp) and you'll plump up for sure.

Also, gaining weight gets easier and easier as you get older, so don't fret!