Mediates tetrahydrobiopterin inhibition of GTP cyclohydrolase 1. This inhibition is reversed by L-phenylalanine. Homopentamer. Forms a complex with GCH1 where a GCH1 homodecamer is sandwiched by two GFRP homopentamers (By similarity). Interacts with GCH1. In epidermis, expressed predominantly in basal undifferentiated keratinocytes and in some but not all melanocytes (at protein level). Belongs to the GFRP family.