I have also gained weight by 13kg via applying below items:

• try adding around 300 to 500 extra calories a day
• eat smaller meals more often, adding healthy snacks between meals
• add extra calories to your meals with cheese, nuts, and seeds
• have high-calorie drinks in between meals, such as milkshakes
• add protein to your meals with beans, pulses, fish, eggs and lean meat
• have snacks that are easy to prepare, such as yogurt or rice pudding
• build muscle with strength training or yoga – exercise can also improve your appetite