The child will get a GC stamped a the US airport provided,
a) the child comes back to US along with the parents on their FIRST trip back to US after the birth of the child. the trip can be taken upto 2 yrs from childs birth.
Basically US Gov interprets it as that the GC holders are US residents. If due to illness or family need , they are away from US and have the baby outside US, then they can be outside aslong as they have to take care of the emergency, but after that the Child has to accompany the parents on their first trip back...
goodluck