I am 135 lbs (+/-) and have a hard time gaining weight and increasing Muscle. What is a good workout regimen?

Also please take into consideration that I am a 4 1/2 stomach cancer survivor. I have only like 35% of a stomach so I am going to need to hear something aside from eat more...because...well I cant. I try so very hard to do that but its not easy for me. I've tried weight gaining shakes and everything. No results are coming from that. Is there a workout regimen for skinny people that I could find?