https://ballotpedia.org/List_of_current_members_of_the_U.S._CongressFrom the present House situation, omitting purple states:
Republicans stand to gain:
Alabama: 2
Florida: 8
Indiana: 2
Kansas: 1
Kentucky: 1
Louisiana: 2
Mississipi: 1
Missouri: 2
NC: 4
Ohio: 5
SC: 1
Tennessee: 1
Texas: 12
Wisconsin:2
For a total of 44 seats
Democrats stand to gain:
California: 9 (California however has legal barriers)
Colorado: 4
Illinois: 3
Maryland: 1
NJ: 3
NY: 7
Oregon: 1
Washington: 2
For a total of 30 seats. If we remove CA, it is 21 seats
This isn't a fight they can win by the numbers alone even with flawless gerrymandering in NY, NJ and CA which would be hard since they would be disenfranchising all the countryside, noting that its far more easy for Republicans to gerrymander compared to Democrats since *all* Dem voters are concentrated in cities (an easy gerrymander)
Your key premise if off, namely that there are no limits whatsoever on gerrymandering, so that a 52-0 Dem California map, or a 36-0 GOP Texas map are feasible. That's not true at all.
First, there are the racial minority seats which are (for now) protected by the Voting Rights Act. As long as the VRA is in place, the GOP can't get rid of the 1 blue seats in MS, AL and SC, as well as with several urban seats in downtown Dallas, Houston, Cleveland, St. Louis etc.
Second, most state constitutions have compactness and community-of-interest requirements for their seats. Even in the absence of a redistricting commission, you can't get arbitrarily ugly and fancy with your maps.
Third, there are many purple or light blue/red states in which the dominant party just doesn't have the votes to create arbitrarily many seats which it can reliably win. Take Georgia, for example: the current 10-4 map is already the best the GOP can get out of that state. Maybe 11-3 would be possible without the VRA, but that's about it. You will never be able to reduce Democratic seats in Atlanta down to zero.