i quickly skimmed this thread so far...

I guess one question i have @Starfall... do you want to gain MUSCLE? or FAT?

If you want to gain muscle, then i second all the suggestions for proteinaceous foods and weight training.

If you want to gain fat, then i COMPLETELY disagree with the protein-heavy suggestions. Protein makes you lose fat. So does exercise and weight training. Those things increase your metabolism more. To gain fat, here's what you do: eat SIMPLE carbs + fat. None of that whole wheat stuff...
What falls into the category? Ice cream, cake, cookies, french fries, etc. Ice cream is a very efficient way I gain weight over the short term without fail (though i'm in a different boat... i need to lose weight)... happens every time.

That said, you should see a doctor if you havent already, to rule out medical disorders such as hyperthyroidism or addison's disease.