I was a skinny guy also. Eat "alot" but cannot gain weight. But my "alot" wasn't really alot since I tend to skip meals. When I follow proper 5 meals a day + consistent weight lifting my weight went up from 49 kg to 72 kg in a span of 18 months.

Now I stop going to the gym my weight drop to 66 kg. I got take protein shakes after lifting also, it helps.