Converts GTP to 7,8-dihydro-D-neopterin 2',3'-cyclic phosphate, the first intermediate in the biosynthesis of coenzyme methanopterin. 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. Belongs to the GTP cyclohydrolase IV family.