I just had a chance to ask the question about how to gain weight on raw food to Dr. Gabriel Cousins M.D. and apart from many of the things already mentioned in this thread he added that you should look at two more things:

1. Taking enzyme supplements for better assimilation of nutrients

2. Make sure you have no parasites in your intestines. In order to know this you should have your stool examined by a specialist. When there are parasites present no amount of food will be able to let you gain weight, because you are just feeding the parasites and they like raw foor just like any other food.