Very little was revealed about the upcoming mode, but you can find the information contained in the FAQ below:
Q:Will Gotham Knights have a multiplayer mode?
A: Yes. Gotham Knights can be played with a friend via the two-player online cooperative (co-op) mode or enjoyed solo at launch. On November 29, 2022, the game will also add Heroic Assault, a standalone mode supporting up to four players in online co-op.
Q:What is Heroic Assault mode?
A: Heroic Assault is an upcoming gameplay mode that is separate from the main story campaign. The standalone mode supports up to four players in online co-op and provides a dedicated arena-like environment with specific objectives to complete and enemies to defeat on each floor (30 floors total).
Q:When will Heroic Assault be available?
A: Heroic Assault will be available on November 29, 2022, as a free update for all Gotham Knights owners.
Q:Will Heroic Assault allow a four-player online co-op for the main campaign?
A: No. Heroic Assault is a separate mode that is not part of the main story campaign.Gotham Knights Site
Gotham Knights is currently available on PlayStation 5, Xbox Series, and PC.