First enzyme in the biosynthesis of tetrahydrobiopterin (BH4) (PubMed:8870645). Catalyzes the conversion of GTP into dihydroneopterin triphosphate (7,8-dihydroneopterin 3'-triphosphate), which is subsequently catalyzed by 6-pyruvoyltetrahydropterin synthase (ptsA) and sepiapterin reductase (sprA) (PubMed:8870645). GTP + H2O = 7,8-dihydroneopterin 3'-triphosphate + formate + H(+) Optimum temperature is 60 degrees Celsius. Cofactor biosynthesis; 7,8-dihydroneopterin triphosphate biosynthesis; 7,8-dihydroneopterin triphosphate from GTP: step 1/1. Toroid-shaped homodecamer, composed of two pentamers of five dimers. Belongs to the GTP cyclohydrolase I family.