I am a father to a baby born with the same Gastroschisis condition. In my experience or to put it more appropriately my partners experience, the amount of milk you will have to produce when your baby is introduced to milk will vary by amount, in the first instance your baby will be fed via TPN (Long Line) then slowly introduced to milk, probably on a scale of 5 to 10ml.
If you have any other feeding issues ask the consultant who will be dealing with your baby and the ICU who will care for your baby after birth.
I also maintain a brand new website dedicated to the Gastroschisis condition which may well have more of the answers you are looking for. Hope this helps