Reductase that catalyzes the conversion of GDP-6-deoxy-D-mannose to GDP-4-dehydro-6-deoxy-D-mannose (GDP-D-rhamnose). GDP-alpha-D-rhamnose + NAD(+) = GDP-4-dehydro-alpha-D-rhamnose + H(+) + NADH GDP-alpha-D-rhamnose + NADP(+) = GDP-4-dehydro-alpha-D-rhamnose + H(+) + NADPH Belongs to the NAD(P)-dependent epimerase/dehydratase family. GDP-6-deoxy-D-mannose reductase subfamily. Extended N-terminus.