One of the best ways to gain weight is by eating more fats.

Now this means "good fats" of course. Fats 1) slow down the emptying of the stomach, so blood sugars do not spike so quickly, and 2) provide extra calories, 3), good fats, typically are in short supply in our food chain in the US. So improving them, is a benefit to the whole body.

Nuts are a good way. Olive oil, flax oil, in salads are another option. Foods like Salmon, or other fatty fish.
You will want to avoid Trans Fats of course.

I wonder if you are having "dumping syndrome" ...IdiopathicPN?
Dumping is where the stomach releases food too soon. When this happens, it is not broken down as it should be so then it would
not provide nutrients for you...hence the weight loss?
Could this be a side effect from your lung medications?

http://www.mayoclinic.com/health/dum...ndrome/DS00715
This can happen in anyone, not just the bariatric patients.

It is something to think about. And I like Glenn's suggestions too.