It is overwhelmingly likely the GOP will flip both chambers. If they don't, they screwed up badly.
The Senate is less likely to flip. Republicans playing Defense there.
Republicans only need a Net Gain of 5 Seats in the House (Right now it's 222-213 DEM). I fall off my Chair if they don't get those 5 Seats regardless what shenanigans D's pull with Redistricting!
Republicans are only playing defense in PA, WI, and NC. States that Biden won by 1 point and a state that Trump won. Democrats are playing defense in Georgia, Arizona, and Nevada, also states Biden won by less than a point and 2 points, in addition to New Hampshire which can be competitive with Sununu. In 2018 Dems easily won in states Trump won narrowly. I don't expect the same thing for the GOP (because they're incompetent) but all it takes is a little wind blow and it's done. There's been no recent midterm where a party successfully holds all those narrow presidential wins, and considering R's just need to net 1, I think it's very likely that happens.