Hi derdoc,
While we each have a genetic predisposition to a general body ‘type’, adding overall calories, especially in the form of higher carbohydrate foods and healthy fats can help with gaining weight (when there isn’t some medical condition causing weight loss of malabsorption). For instance, if you normally have one serving of a starchier carbohydrate (sweet potato, quinoa, gluten-free oats, etc.) per day, increase to 2x/day. If you normally have one ounce of nuts in a serving, have two.
Additionally, be sure to eat slowly and if you are not doing so already consider using digestive enzymes and probiotics (or eat probiotic-rich foods like sauerkraut) as well to make sure you are absorbing more of what you are eating. Finally, maintaining muscle mass with adequate protein (minimum of .8 g per kg of body weight for a healthy person, and varying need with activity level) and resistance exercise is essential as well.
Hope that helps! -Coach Zoe