Hydrolyzes glucose-6-phosphate to glucose in the endoplasmic reticulum. Forms with the glucose-6-phosphate transporter (SLC37A4/G6PT) the complex responsible for glucose production in the terminal step of glycogenolysis and gluconeogenesis. Hence, it is the key enzyme in homeostatic regulation of blood glucose levels. D-glucose 6-phosphate + H2O = D-glucose + phosphate Carbohydrate biosynthesis; gluconeogenesis. Defects in G6PC1 are the cause of glycogen storage disease Ia (GSD-Ia); also known as von Gierke disease. GSD-Ia is characterized by hypoglycemia and hepatomegaly. Belongs to the glucose-6-phosphatase family.