Is there a specific diet before/after gastric bypass?

I am 46 and during the pregnancy with my second child I gained a lot of weight and even after birth I constantly gained around 2 pounds per month. I do not have an appetite nor do I eat large portions. I plan to have a gastric bypass surgery later this year and I would like to know if there is a recommended diet before and after the surgery and also how long after the surgery I would have to stay on this diet.