The cause of hyperuricemia and gout in glucose-6-phosphatase deficiency is:

Type I glycogen storage disease (Von Gierke's Disease) is an autosomal recessive disorder caused by glucose-6-phosphatase deficiency in liver, kidney, and intestinal mucosa. Purine overproduction and hyperuricemia in von Gierke disease (glucose-6-phosphatase deficiency) occurs secondary to enhanced generation of the PRPP precursor ribose 5-phosphate. An associated lactic acidosis elevates the renal threshold for urate, elevating total body urates. Ref: Rodwell V.W. (2011). Chapter 33. Metabolism of Purine & Pyrimidine Nucleotides. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.