We often advise people to avoid Snacking but if you need to gain weight adding a few high calorie snacks throughout the day should help.
Things like cheese, olives, nuts and avocado, some nuts can be quite high carb so make sure you choose the lower carb ones.
Several cups of coffee with a generous serving of cream would also be a delicious way to increase your calories (I think I've gained a couple of pounds just writing this)