A derelict shopping center that has long since been abandoned and forgotten to time, the Gotham Mall is the replacement hideout that Harley Quinn and her crew took over after they were evicted from Ivy's apartment.
Long before the events of the series, Sy Borgman hid his sister, Mirielle Borgman in the basement of the old abandoned Gotham Mall to keep her safe after she was mutated into a Human-Octopus Hybrid. He kept her there in secret for many years always returning to feed her live goats. He also had a crematorium installed in a pizza oven and used the place to dispose of several bodies over the years including some of which that weren't dead yet.
When Sy found Harley Quinn, Poison Ivy, Doctor Psycho, Clayface and King Shark lying on the ground, seemingly dead and wearing 'Suicide Squad' shirts in Ivy's apartment he thought he had stumbled upon a suicide cult and decided to get dispose of the bodies. With help from former colleague Golda he moved the bodies to the Mall to have the cremated but they woke up just in time before they got burnt alive. As compensation for almost killing them, Sy agreed to gift the Mall to Harley as the new hideout for her crew in exchange for him getting to join in on some of their heists.
After Harley betrayed the crew by going back to the Joker and neglecting her crew members in "A Seat At The Table", they all quit the crew, thus leaving Harley and Sy as the solve residents of the Mall. However, they all ended out rejoining the crew in the finale after Harley earned back their trust and they returned to the mall.