Catalyzes the hydrolysis of dGTP into dGMP, which is needed among other for the first step of biosynthesis of dZTP (2-amino-2'-deoxyadenosine-5'-triphosphate). dGTP + H2O = dGMP + diphosphate + H(+) dATP + H2O = dAMP + diphosphate + H(+) Purine metabolism. Belongs to the Caudovirales dATP/dGTP diphosphohydrolase family.