Catalyzes the hydrolysis of the N-glycosidic bond in the first two intermediates of riboflavin biosynthesis, which are highly reactive metabolites, yielding relatively innocuous products. Thus, can divert a surplus of harmful intermediates into relatively harmless products and pre-empt the damage these intermediates would otherwise do. Has no activity against GTP, nucleoside monophosphates or ADP-ribose. Catalyzes the conversion of GTP to 2,5-diamino-6-ribosylamino-4(3H)-pyrimidinone 5'-phosphate (DARP), formate and pyrophosphate. 2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)-pyrimidine + H2O = 2,5,6-triamino-4-hydroxypyrimidine + D-ribose 5-phosphate 5-amino-6-(5-phospho-D-ribosylamino)uracil + H2O = 5,6-diaminouracil + D-ribose 5-phosphate GTP + 4 H2O = 2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)-pyrimidine + formate + 3 H(+) + 2 phosphate Binds 1 zinc ion per subunit. kcat is 137 sec(-1) with 2,5-diamino-6-hydroxy-4-(5-phospho-D-ribosylamino)pyrimidine as substrate. kcat is 0.07 sec(-1) with 5-amino-6-(5-phospho-D-ribosylamino)uracil as substrate. Cofactor biosynthesis; riboflavin biosynthesis; 5-amino-6-(D-ribitylamino)uracil from GTP: step 1/4. In the N-terminal section; belongs to the YbiA family. In the C-terminal section; belongs to the GTP cyclohydrolase II family.