The best way to measure forecast accuracy is MASE (Mean Absolute Scaled Error). It was proposed by Rob Hyndman in 2006 and compares forecasts to the naive or the "no change" prediction. The chart below shows MASEs for four different Germany GDP forecasters.
Many sales forecasts have MASEs well below 55%. If this is true for your Germany sales forecast, then there is a low probability that your forecasts will improve by adding Germany GDP.