Requires Mg2+. This enzyme catalyses the hydrolysis of the imidazole ring of guanosine 5′-triphosphate, N7-methylguanosine 5′-triphosphate or inosine 5′-triphosphate. Xanthosine 5′-triphosphate and ATP are not substrates. It also catalyses the hydrolysis of diphosphate to form two equivalents of phosphate. Unlike GTP cyclohydrolase II (EC 126.96.36.199), this enzyme does not release formate, but does hydrolyse the diphosphate from GTP to phosphate.