jakechip:
Are there any ways of gaining fat that wouldn't be detrimental to your health? What foods would you guys recommend?
Very simply, eat more good food. Maintain a balanced diet (ie: making sure you are getting veg & fruits as well as everything else), but up your portions. A LOT, if desired!
The danger is from eating crap foods (ie: lots of processed sugars, lots of trans fats) like in processed and fast food. Now, I eat those, too, but they don't make up a big percentage of my food intake.
If you push your limits and stretch out your stomach, you will be hungry all the time - that is the key to increasing your food intake, in my opinion.