Gastroschisis is the exposure of abdominal contents due to congenital failure of the anterior abdominal wall to close. Whereas, omphalocele is the persistent herniation of the bowel (midgut) into the umbilical cord. Normally, it's a physiological herniation to allow space for a developing liver. The midgut loop then returns back during the 11th week or so. So in gastroschisis, the contents won't have any covering whereas in omphalocele, the contents (bowel) will be covered with peritoneum and amnion of the umbilical cord. (You see exposed abdominal contents with a transparent covering, that's omphalocele)