Enables GTP cyclohydrolase I activity and GTP cyclohydrolase I regulator activity. Involved in several processes, including compound eye pigmentation; preblastoderm mitotic cell cycle; and pteridine biosynthetic process. Located in cytoplasm. Is expressed in several structures, including adult head; imaginal disc; larval ventral nerve cord; and oocyte. Human ortholog(s) of this gene implicated in several diseases, including BH4-deficient hyperphenylalaninemia B; diabetic angiopathy; movement disease (multiple); sickle cell anemia; and type 2 diabetes mellitus. Orthologous to human GCH1 (GTP cyclohydrolase 1).