How fatal and painful is it if my belly gets stabbed/ripped open?

It depends on what exactly gets cut or punctured, of course, but my understanding is that without proper surgical intervention it can be an exceptionally slow and painful way to die.

The organs in the gut are mostly intestines. You're not going to die just because they've spilled out, but you're going to be bleeding pretty badly and if whatever caused them to spill out is still around then you're pretty screwed.

The bigger problem is that it's unlikely they've just spilled out, they're probably also sliced open. Now you're in serious trouble, because there's lots of blood in there so now you're bleeding really badly. You've also got blood and the content of your digestives system mixing together, and that means some very nasty bacteria which are normally safely contained now have access to your blood.

I suspect the most likely dangerous situation is a stab wound. In that case you'll probably experience internal bleeding. There are no shortage of places for blood to go inside your body around there, including into your digestive system. I don't think there's anything much to stop blood from flowing endlessly into there, and you could bleed to death even if the external wound doesn't look like it's bleeding all that badly.

In summary, getting stabbed in the gut will contaminate your blood and lead to potentially endless bleeding which can't be treated with bandages because it's inside. Even if you avoid bleeding to death, you're probably going to die from a massive infection