Gaining

Eat low fat versions of everything like low fat milk and sherbet, cake instead of cookies, chicken instead of hamberger.

My advice falls into two camps:

1. Diet
2. Exercise

As mlc says, it's perfectly possible to be healthy without being skinny, just as it's possible to be skinny and unhealthy.

Diet

Obviously you need to be a bit careful if you have a history of cardiac disease in your family. However, I would emphatically *not* recommend low fat everything. The cholesterol-heart hypothesis is not as solid or simple as it is made out in the media or by many doctors. I would recommend doing your own research on the matter, but you might want to start here.

So, it's important that you eat well - meat, fish, fats (not industrial oils like vegetable oil), vegetables, fruits and nuts (but don't go crazy on these). As mlc indicates, the nutrient profiles (and nutrient bioavailability) of vegetables change with cooking, but not necessarily negatively. If you're eating lots of food I wouldn't go out of your way to eat loads of "superfoods" as so long as you avoid crap you'll be getting a good amount of nutrients anyway. Superfoods tend to be expensive and a bit overhyped, but if you like them then fair enough.

In a nutshell - avoid processed food.

Exercise

Exercise is the key to health at a higher weight. It's not so much about jogging, though. To be honest, I'd never recommend running to people who are overweight, as you'll likely get injured unless you're very strong and have good technique. If you're serious about your health, make sure to build movement into your everyday life - walk lots, cycle instead of drive etc.

The other I'd suggest is do weights. Not silly bicep curls with tiny weights so you don't get "too bulky", but proper functional compound movements like squats, deadlifts, lunges, press-ups, pull-ups etc. Higher lean mass (i.e. muscle) has been shown to correlate with greater health and life expectancy. It will also help maintain or enhance your quality of life.

In a nutshell - move lots, and lift heavy stuff once or twice a week.

If you dont eat healthy or exercise you will be more inclined to develop high blood pressure, high cholesterol, heart disease, bowel cancer etc.
that has absolutely nothing with the amount of fat tissue your body has.

So if you ate a healthy diet, exercised and then used a low sugar weight gain shake every day to bring you over your caloric requirements or something like that you would be gaining weight without making you susceptible to those diseases.

However, and this really seems to be left out of alot of discussion on fat health, is that there are disorders out there that are caused directly by the fat tissue, things like diabetes, meralgia parathesia, hernias, etc. Obviously not all fat people will develop these but I do think they need to be mentioned.