Life is seldom as simple as it looks.
If you have ever watched someone taking their last gasps of air dying of cancer, you would realize that the quiet end that comes with dehydration is not the worst fate.
Everybody needs to make their own choices but if my end were cancer I would choose to not receive a gastric tube or IV fluids.
The real question comes when you already have the tube inserted, and it's doing exactly what it's supposed to do (supplying you with nutrition which you can assimilate), and it is not burdensome in itself, but somebody wants to remove it for the purpose of starving you to death.
That' a different matter altogether.