To gain 20lbs you have to eat a ton of quality food. Good low glycemic carbs (brown rice, fruits, whole grain breads) and quality proteins.
As long as you are doing the nutrition side of it right I would just make sure you stay consistent with a high calorie weight gainer. My choice is definitely PVL by Mutant Mass and maybe an a quality vitamin pack. Some of the best ones even come with a fish oil (omega 3) in them.
In the end though it will all come down to how consistent you are. (Training+eating+supplements)
Good Luck!