To the OP:
Is your GD the newest facelifted model? How much RPM do you see when you drive 100 km/h at 6th gear?
According to technical specs found at hyundai.fi , the facelift GD CRDi has significantly taller gear ratios compared to the earlier models.
Non-facelift 5d Non-facelift Wagon Facelift 5d and Wagon
1st 3.636 3.636 3.769
2nd 1.962 1.962 2.040
3rd 1.189 1.189 1.189
4th 0.844 0.844 0.844
5th 0.702 0.702 0.702
6th 0.596 0.596 0.596
R 3.583 3.583 3.583
Final 3.471 3.941 3.250
2000 RPM 115,5 km/h 101,7 km/h 123,4 km/h
@ 6th
100 km/h 1731 rpm 1965 rpm 1621 rpm
@ 6th
However I can't say how much of this is correct. From elsewhere I have seen bit different values, so those gear ratios may also vary in different countries. I can at least confirm that my non-facelift GD Wagon does about 2000 rpm @6th @100 km/h, which suits me just fine.
I'm having hard time to believe if facelift model really has only 1600 rpm at 100 km/h!? IMO overly tall gear ratios is a great way to ruin the driving experience for the sake of few grams of CO2.

Those who are towing caravans would not be happy either.