Few are saying the GOP is a lock to hold the House. However, due to gerrymandering the national Dem House vote can't fall more than a few points behind Hillary for a flip, and few GOP Congresscritters are as unpopular with their constituents as Trump is.
More than a few ought to say the GOP is a lock. Even with a ten point swing in House races and factoring in Democrats doing even better than that in open seats no longer defended by a Republican incumbent, you just barely get close to a tied House. We're not seeing in the polls a ten point presidential swing, let alone in the downballot races. The Democrats should make gains, but their focus for this election needs to be the Senate and possibly some State Houses, if they are in a condition to be flipped.