Life expectancy doesn't change due to Gastroparesis unless the person is in really bad shape and about to die due to some complications like malnutrition that is not fixable or an infection, just as examples. However Gastroparesis does not change life expectancy itself despite being the cause that complications arise.
Complications like malnutrition, dehydration, infection, etc can lead to death or make it seem like someone may lose their battle when they actually recover unlike the person who passes away. Some medications may lead to other illnesses that can bring down life expectancy if you develope complications.
Gastroparesis has life threatening complications but if it is a manageable case, it is very possible to live a full (length wise) life with gp as far as we are currently aware. Be cautious with gp as it is very easy to dehydrate.
Many people I know about personally have passed and their age ranges from babies, toddlers, kids, teens, young adults all the way to older adults. It doesn't have an age expectancy. Anyone of any age can survive or pass depending on circumstances.
That can be scary but remember if you do what you need to do you can expect a long life with good reason. Don't expect to die because while many people with g.p. have been passing its circumstantial.