I would first of all take a look at the structure of vitamin K. It has a 1,4-quinone moiety that is the redox active portion of the molecule. This is its oxidized form and the hydroquinone is the reduced form. So upon carboxylation of glutamate, the reduced cofactor becomes oxidized, meaning that one of the substrates must be reduced in this process. As you say, a proton is removed from the glutamate molecule, but this isn't an redox process, it's simple acid/base chemistry. So what other molecules are reacting? How about the carbon dioxide? It is going from the highest possible oxidation state for a carbon atom (4 bonds to oxygen) to lower oxidation state carboxylic acid. The hydroquinone provides the electrons for this redox process to take places, and is converted to the quinone form. Another redox enzyme, likely with an NAD(P)H cofactor, will then reduce the quinone form to regenerate the reduced cofactor.
