Is there an app thats like a weight loss aid, but instead for gaining weight?

I know its weird to see a question thats the opposite of societys wishes but i have a BMI of 16.6, which a healthy one is around 18.5. I know they have iphone/android apps that help people who are trying to lose weight, which include a tracker which foods youve eaten and how much exercise youve done. I'm looking for an app thats similar, but for gaining weight
I need something like that for assistance, your help is very much appreciated

Use the same. Put your goal weight. Determine calories based on the recommendation. Or skew the data to equal the calories you want. So, if you want two thousand, say you weigh three hundred and want to weigh two hundred ninety.