Catalyzes the reduction of arsenate [As(V)] to arsenite [As(III)]. Does not constitute the major arsenate reductase in cells: essential only in the absence of ArsC (AC P74313). [glutaredoxin]-dithiol + arsenate + glutathione + H(+) = arsenite + glutathionyl-S-S-[glutaredoxin] + H2O Belongs to the ArsC family.