Getting shot in the stomach is not always fatal if you get medical treatment as soon as possible and by soon as possible I mean right then if possible.
So get an ambulance on the way right away after being shot if you can because it could be fatal if not treated soon enough.
When dying from a gunshot to the stomach it is usually a slow painful death if you do not get treated and the death can take only a few hours to a few days.
It all depends on how bad the gunshot to the stomach is and if it tore into any other organs.
But always seek medical attention for any gunshot wounds because they can be fatal pretty soon.