There are two types of weight gain.
1. Increase in muscle mass..... You can achieve this by having three complete home made meals, you can take extra helping of chapati, dals and vegetables. Take milk, curds, eggs, nuts, dry fruits on regular basis. You can take potato or banana also to increase the weight.
2. Increase in fat mass..... If you increase intake of fat foods and sweets to increase weight, you will gain fast mass more than healthy weight.. Many a times increase in fat mass leads to hormonal disturbances and hypertension and heart diseases.