When you eat gone off sausages the gone off sausages will cause you to get a stomach ache, severe stomach pain and possible food poisoning.
If the sausage has actually gone off and the sausage is bad and smells bad or tastes bad then you should throw the sausage out and not eat it.
In most cases if you just happened to eat some sausage that has gone bad and you only ate one bite of the sausage or a few bites of the sausage then at most you'll likely just get a stomach ache and maybe non life threatening food poisoning.
You may experience vomiting along with the stomach pain.
Most cases of food poisoning go away on their own without treatment but if you think you may have botulism you should see a doctor just in case.
Botulism can be deadly but simple food poisoning usually goes away on it's own.