Catalyzes the GTP-dependent successive addition of two or more gamma-linked 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) or polyglutamated F420 derivatives. 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. Cofactor biosynthesis; coenzyme F420 biosynthesis. Homodimer. Belongs to the CofE family.