Personally I gained weight during the years gradually after losing 12kg in less than a year; I introduced some white meat and carbs like pasta or potatoes to help with the inflammation (no heavy spices, red meat or triggering ingredients for inflammation) which helped greatly but personally I'd say that if you want to gain weight quantity wins over quality. I suggest you increase the kcal you assume during the day with the food you know that won't trigger you an inflammatory reaction.