Some Republican success, it is true, was down to recent shameless efforts to gerrymander the nation's congressional boundaries. But that does not fully account for their roughly 40-seat majority. Besides, 30 states now have Republican governors, though state borders cannot be gerrymandered.
The first assertion seems a bit dubious, as fewer votes were recorded for Republican House candidates than for Democratic ones, by a slim margin: 53.4 million to 53.9 million. It may well be that it is state-by-state variation, rather than gerrymandering, which accounts for the discrepancy. But, certainly in a proportional-representation scheme we would face a Democratic majority (or perhaps a minority government in coalition.)
The second assertion just assumes that previous generations' border-drawing exercise is somehow natural. :) I can't find a quick source for the total population governed by Republicans vs. Democrats, but I wouldn't be surprised if the population count (or vote count) showed a closer or opposite result.
But talk of a "mandate" is generally nonsense to begin with, so I guess you might as well try to justify gridlock if that's your starting point.