Catalyzes the GTP-dependent successive addition of two L-glutamates to the L-lactyl phosphodiester of 7,8-didemethyl-8-hydroxy-5-deazariboflavin (F420-0) to form coenzyme F420-0-glutamyl-glutamate (F420-2), with a gamma-linkage between the two glutamates. Cannot use F420-2 as substrate to add more glutamates. Exhibits maximum activity with GTP, compared with UTP (66%) and dGTP (25%); with ATP, only F420-1 is observed as the product; CTP and TTP support no activity. GTP + L-glutamate + oxidized coenzyme F420-0 = GDP + H(+) + oxidized coenzyme F420-1 + phosphate GTP + L-glutamate + oxidized coenzyme F420-1 = GDP + H(+) + oxidized coenzyme F420-2 + phosphate Binds 2 divalent metal cations per subunit. The ions could be magnesium and/or manganese. Monovalent cation. The ion could be potassium. Optimum pH is 8.5. Optimum temperature is 60 degrees Celsius. Thermostable. Retains 70% of its activity after heating at 80 degrees Celsius for 15 minutes. Cofactor biosynthesis; coenzyme F420 biosynthesis. Homodimer. Belongs to the CofE family.