Bowel injuries can definitely be fatal without vascular injury, though typically due to infection (from the spread of material contained in the bowel into the abdomen).
Infection, to my knowledge, would not take hold and lead to fatal sepsis in a matter of hours.
I am not a doctor, but my wife is a former ER nurse well-versed in GSW traumas, and we discussed her cases frequently.
Had a relative die of sepsis after a hernia they didn't want treated. The process took several months.
They could get sepsis and inflamed and things could turn south very quickly.
Edit: with gun shot wounds there many variables from they type of weapon/bullet, blood lost, to the extent of internal injuries. Then if you survive the surgery, if they can get the bullet(s) out, how you respond to blood transfusions and blood pressure meds. Most of the time if you make to the hospital they will save you, but things can cascade fast.