Nonsense. Maybe Republicans have been more successful in gerrymandering, but both parties have engaged in this practice. That's why there are so many "reliable" Republican and Democrat seats.
In certain places, you know for certain that the 'R' or 'D' candidate is going to win. Incumbents from both parties typically have a 90+% re-election rate.
If Democrats are so much more popular, why aren't they able to maintain majorities and governorships in state governments and re-district to their own advantage? Are they too principled to use this tactic?
That's why the TEA Party was such an excellent movement. They managed to oust incumbents who had little chance of losing a general election. Impressive achievement for a bunch of old white people. How many incumbent Democrats have progressives and socialists managed to defeat in the last 10 years? And it's not like there aren't plenty of 'D's whose only appeal to the left is that they are marginally better than Republicans.
Note that with a few rare exception, I hate both of these scumbag parties and have rarely voted for either.