Bifunctional enzyme that catalyzes the GTP-dependent successive addition of multiple gamma-linked L-glutamates to the L-lactyl phosphodiester of 7,8-didemethyl-8-hydroxy-5-deazariboflavin (F420-0) to form polyglutamated F420 derivatives, and the FMNH2-dependent reduction of dehydro-F420-0 to form F420-0