This picture shows a Republican National Committee member's own calculation of what would happen if voting districts were determined by independent commissions instead of partisan legislatures. The long and short of it, at least according to him: Democratic Party gerrymanders like New York would stay Democratic, while Republican gerrymanders would flip blue.
The reality of this is more complicated than this map implies, but sometimes you just have to let them go with their own craven cynicism, simply because it's so revealing about their politics.