Walking is always allowed, and as you heal you can go full out walking as long as you're not experiencing any pain.
There are a lot of nerves in and about the surgical site, and they can refer pain to many different places. Gas puts pressure on the inflammed nerves, and that is likely the culprit. It's so strange that the surgery at the stomach/esophagus junction can cause shoulder pain. When I discussed it with my surgeon, he said just when he thinks he has heard all the possible post-op symptoms, someone comes up with a new one!
Glad you've joined the forum! Keep those questions coming.