Catalyzes the conversion of GDP-4-dehydro-6-deoxy-D-mannose to GDP-6-deoxy-D-talose. GDP-6-deoxy-alpha-D-talose + NAD(+) = GDP-4-dehydro-alpha-D-rhamnose + H(+) + NADH GDP-6-deoxy-alpha-D-talose + NADP(+) = GDP-4-dehydro-alpha-D-rhamnose + H(+) + NADPH Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Belongs to the NAD(P)-dependent epimerase/dehydratase family.