To add to this I have just got into weightlifting but I mostly train doing bench press etc other dynamic lifts
Also have very high metabolism as I eat about 3500-4000 calories a day and don't really gain
Gaining can be tough. I think the trick is finding calorie dense foods that are easy to eat. Pistachios are a great example. any nut really. Try adding some peanut butter sandwiches to your normal diet or even a casein protein supplement at night. Never get hungry!