If you are undergoing a gastric bypass surgery, there are things to remember. It is not your ultimate cure to obesity related problems. You have to consult with a doctor on a regular basis and basically learn to change your lifestyle habits. I would say actually try consulting a professional for that kind of stuff before you even get started with the idea of using that kind of procedure.
But also consider the fact that while you automatically eat less, there are those that literally go crazy, stuff themselves until they throw up, doing it again and again until they pop the stitches, doing nobody any good. It is a major change for anyone and it can be scary. In addition, you could easily suffer from different vitamin deficiencies leading to the fact that you need to actually take a multivitamin along with an otherwise balanced diet during use.
There will be no sneaky fatty foods, as this will only make it worse, especially with so little that you can actually eat throughout the day. If you suffer from deficiencies, it will interfere with your weight loss and you will also suffer other obvious side effects. So before even considering this idea, think again.