My weight fluctuated between 10 and 22 stone for about 30 years so I have learned what makes me lose and gain weight.
To gain weight, all I would need to do is eat lots of high carb food with high fat. Junk food, basically. Potato wedges or nachos with lots of cheese and sour cream and high sugar chilli sauce. Pizza. Ice cream with hot fudge sauce. Baked products like muffins, slices and cake. Toasted cheese sandwiches, pita bread with cranberry and brie, even salads with croutons and high sugar/fat dressings.
Another option is supplement drinks like Ensure that have high carbs.
Good luck with your weight gain!