Hello.If you want to gain weight then you should join a gym and consult this with a trainer.you need protein so for it you should drink protein shake regularly and you can eat boiled potatoes.You should eat 1 egg regularly for your weight gain.
I do not recommend protein shakes, as they are processed and contain a lot of sugars and whey (a dairy product). If you are going to follow strict paleo then potatoes are out, however many paleo followers feel that potatoes are paleo-friendly (just as some feel that dairy products are ok). Depends on what you feel is right for yourself. I personally do not eat potatoes (other than sweet potatoes as mentioned in an earlier post) russet, yukon golds, etc... tend to make me feel slow and lethargic, that's my personal experience. Good protein sources for after a workout are egg, fish (starkist tuna is awesome!), chicken, beef (Fatty cuts are the best, try chuck or a steak with the marrow bone).
I would stick with whole foods rather than processed junk that preach weight packing abilities. Natural is always the best choice.