What kind of weight do you want to gain? Water will come along with stored carbs (glycogen) in the body, especially if you've been lower carb for a while, but it might not be the type of weight you want. Do you want to build up some muscle mass? That will require strength training and protein to build and then more strength training and fats or carbs to maintain (it'll need energy or else it'll starve and go away--the increase metabolism we all strive for). Or do you want to regain some body fat (not necessarily a bad thing if you are too low)? Eating healthy fats is probably the most efficient way to go about it.
You should be warned that while this is based on what I believe is sound nutritional info, I can't say I've ever had the opportunity to put this into practice.
Well, at least on purpose.
Anne