The enzyme deficient is Glucose 6- phosphatase, the enzyme required by the liver to release glucose in the blood.
if you give fructose or galactose, to these patients, first these sugars need to be converted into glucose by the liver & released into the blood. This would required the enzyme G-6-Phosphatase, which is deficient, so giving these sugars is not going to inc. the blood glucose.
Giving glucose to any body with or without von Gierke's should increase their blood levels.