Catalysis of the reaction: GDP-6-deoxy-D-mannose + NAD(P)+ = GDP-4-dehydro-6-deoxy-D-mannose + NAD(P)H + H+. In the reverse reaction, a mixture of GDP-D-rhamnose and its C-4 epimer is formed.
GDP-6-deoxy-D-mannose:NAD(P)+ 4-oxidoreductase activity