How to gain weight ?

Load up on carbs and heavier proteins, like beef and pork. Talk to a nutritionist. 

Healthy fats, increasing calories, and good healthy complete proteins help me keep weight on (I used to be 110 and am now around 122-124 lbs). Plus a couple years ago I started using free weights when working out, and limit my cardio to max. 15 minute warmups, then ensure I have protein immediately after a workout to keep calories up. Good luck 😊

Eat your same healthy meals but in bigger portions. I suggest still choosing nutrient dense foods to fuel your body but increasing the amount to reach a caloric surplus. Also, don't do any cardio perhaps! 

When I was trying to gain weight I was drinking Ensures like they were going out of style.