Mediates tetrahydrobiopterin inhibition of GTP cyclohydrolase 1. This inhibition is reversed by L-phenylalanine (By similarity). Homopentamer. Forms a complex with GCH1 where a GCH1 homodecamer is sandwiched by two GFRP homopentamers. Interacts with GCH1 (By similarity). Belongs to the GFRP family.