If the gerrymandering is done most efficiently, the gerrymandered side only wins by a few percent. So if the historically non-voters come vote for the underdogs, they can flip those districts.

That’s why there was no red wave in 22, and why the fascists are trying as many other methods to disenfranchise voters they don’t want voting (no food or water in long lines created by reductions in polling places and their hours, purging voter rolls, mail in voting bans, etc).