The Seagate Amusement Park was an off-shore oil rig that was turned into an amusement park that served as one of the Joker's bases of operations. It was new in the Arkhamverse, and first appeared in Batman: Arkham Knight, but only in the background. It officially appeared in Batgirl: A Matter of Family.
A Matter of Family Incident
3 years before the events of Batman: Arkham Asylum, the Joker and Harley Quinn, along with a large following of their henchmen, abducted Commissioner Gordon, and announcing that if Batman attempted to intervene, they would kill him instantly. Believing it her personal duty to ensure her father's freedom (as well as to prevent Batman from having to go), Batgirl, assisted by the new Robin, Tim Drake, made her way to Seagate on a rescue mission.
Unbeknownst to them, that had been Joker's plan all along. Insisting that their allegiance to Batman interfered with Joker's and Batman's "crazy little game", it was his intention to eliminate the duo and presumably the commissioner too.
Eventually, the Joker's plan was foiled when Batgirl and Robin dispatched the majority of Joker's Henchmen and engaged the Clown Prince of Crime, Harley, and their remaining forces in a hand-to-hand brawl. Soon afterwards, the GCPD were called in for backup, though were they unable to detain Joker before he made a leap from the edge of the Seagate platform and was carried away by a parachute.
That was one of the last major acts in Batgirl's career before she was paralyzed by the Joker.
Arkham Knight Incident
During the events of Arkham Knight, Seagate Amusement Park could be seen best from Miagani Island, and sat on the coast of a peninsula in the Gotham City Bay. In a state of complete disrepair after its abandonment by the Joker's Henchmen, it was empty, like much of the quiet outskirts nearby.