I have to share ma xp. I'm a hard gainer like you. But I gained 2.125 kg in just 1 week. The first key to gain weight is to eat constantly like 6 times a day. The food you eat depends on your budget but the content should have higher carb and fat. Don't forget protein too. Generally I consumed around 3000 calories per day. But you have to progress don't try to eat 3000 in the very first day. To increase your calorie consumption use shakes. I would recommend the organic one like the beso minamn stuff. The other most important thing is working out. And be consistent in all of the above. I will guarantee you will put some weight. Good luck