http://nutrition.about.com/od/tryingtogainweight/a/gainehalthyweig.htm
this site has the information you need. It's basically the opposite of losing weight meaning you need to eat more calories than you burn during the day. While those losing weight need to burn more calories than they eat.
Good luck with your weight gain and surgery.

Hi, for you to put on weight you will need to increase your carb intake so foods like rice, sweet potatoes, pasta, bread, you can also get in extra calories with a weight gainer protein shake which is high in carbohydrates and also contains protein. Also weight training is a brilliant way to put on more weight.

To find out more tips on weight training and eating plans for putting on weight visit http://www.sculptrexfitness.co.uk

but won't weight training make burn calories i do not want to do that
I have heard from someone that musule weighs more than fat is that true?

Yes this is true muscle weighs more than fat, which I suggested you took up weight training with a higher carb intake, in regards to burning calories all types of exercise burns calories, but the benefit with weight training without doing any cardio is you will build and grow when done in conjuction with a high calorie intake.

I have a feeling that gaining 40lbs in a few months would be as tough or maybe tougher than someone trying to lose that much weight in the same period of time.
While all the advice you have received is good, I think I would ask your dr about seeing a nutritionist or dietitian to ensure that your gain is a healthy one.
As for the exercising part, maybe seeing a trainer at a a gym for some help would be a good idea.
Your goal won't be easy so help from some professionals may make it a bit easier for you.
If I could figure out how to do it, I would give you 40 of my pounds.

good luck to you.