It's not just gerrymandering, I believe, but actual demographic patterns. It's mostly urban districts that are 80-90% Democratic. And yeah, I know about creating black-majority districts which both help black candidates win primaries and create more GOP-leaning districts overall. But right now, any "fair" drawing of Congressional districts that tried to represent natural communities or geographic areas would lean GOP, just like the electoral college seemingly leans Democratic right now.
Either way it would have to be a pretty big swing. I wouldn't put it past the GOP to shoot themselves in the foot that hard, but it's nowhere near guaranteed.