False equivalency. Not only do fewer states Gerrymander in favor of Democrats, the degree to which that small number shift is MUCH smaller. For example, Washington, Oregon, and California have the blue shift statewide to lock out ALL Republican representatives to congress with Gerrymandering. Washington, instead, is split 6:4, Oregon is 4:1, and California is 39:14. Just those three states putting a full-court-press Gerrymander in place would shift the House of Representatives by 19 seats.
Contrast that with the two most-Gerrymandered states with a 50:50 split of (R) and (D) voters, North Carolina and Pennsylvania. NC has a 10:3 split in favor of (R) and Pennsylvania has a 13:5 split.
Gerrymandering is not rocket science. Any state leaning (D) by 10 points could easily game the system like North Carolina does. Actually, the populous Western states could do it more easily, as there are vast areas of very low population density that can be merged with a few square miles of high-density, liberal-leaning urban area to form a congressional district that looks OK on a map (compared to the squiggly lines you find elsewhere) but could be counted on to vote (D). So it’s not a matter of Republicans being better at it, and those dumb Democrats not being able to figure it out. 
In several states, as others have already pointed out, Democrat majorities have written it into state law to make Gerrymandering illegal or even requiring a rebalancing of districts to keep the representation of the state close to the state-wide political demographics. Based on this evidence, one could conclude that there is one party that is out to win at all costs, while the other sees politics as a service to the people. Who knew? 
