As for gaining weight naturally...

If you are truly an ecto... then you dont have to worry about gaining excess body fat.

And you are not eating enough.

Eat more.

Dont be afraid to splurge.

If you feel like youre getting soft and not big... up the intensity of your training.

Add in more volume (more sets, reps)

Lessen your rest periods.

Keep that heart rate up and thatll allow you to eat more.