Converts GTP to 7,8-dihydro-D-neopterin 2',3'-cyclic phosphate, the first intermediate in the biosynthesis of coenzyme methanopterin (By similarity). Involved in archaeosine (G(+)) and folate biosynthesis (PubMed:18931107, PubMed:19478918). GTP + H2O = 7,8-dihydroneopterin 2',3'-cyclic phosphate + diphosphate + formate + H(+) Binds 1 Fe(2+) ion per subunit. Cofactor biosynthesis; 5,6,7,8-tetrahydromethanopterin biosynthesis. Homodimer. Cells lacking this gene are auxotrophic for thymidine (dT) and hypoxanthine (PubMed:19478918). tRNAs of the mutants lack archaeosine (G(+)) (PubMed:18931107, PubMed:19478918). A slight growth defect is seen in mutants grown at 45 degrees Celsius in Hv-YPC (yeast-peptone-casamino acids) medium supplemented with dT (PubMed:18931107). Absence of pantothenate from the growth medium of the mutant cells results in a slow-growing phenotype (PubMed:19478918). Belongs to the GTP cyclohydrolase IV family.