Converts GTP to dihydroneopterin triphosphate. Is not active with GDP, GMP, ATP, CTP or UTP as substrate. GTP + H2O = 7,8-dihydroneopterin 3'-triphosphate + formate + H(+) kcat is 2593 min(-1). Optimum pH is about 4.0. Activity strongly decreases above this value. Optimum temperature is about 35 degrees Celsius. Activity strongly decreases above this value. Cofactor biosynthesis; 7,8-dihydroneopterin triphosphate biosynthesis; 7,8-dihydroneopterin triphosphate from GTP: step 1/1. Homohexamer. Belongs to the GTP cyclohydrolase I type 2/NIF3 family.