Milk is a great choice for hardgainers as well as for those looking to gain size and strength. (fitnessandpower.com)
The majority of hardgainers need at least 20 calories per 1 lb of body mass to start gaining weight. (fitnessandpower.com)
You also asked if you need to be on a «hardgainer diet» to gain muscle, but it's not clear to me what exactly you were referring to with this term. (kingofthegym.com)