Gotham Knights is going to have a free 4-player co-op mode according to the latest news. This mode will be called “Heroic Assault”, and it will be a standalone game mode. Release date for this content is announced as 29th of November.
This separate mode will initially support two players. It will feature an arena-like map instead of a story campaign of its own, and you will have unique objectives and enemies in this mode. Players will have to ascend through 30 floors, defeating various enemies during the challenge. This mode will not have any sort of microtransactions, so everyone will be completely able to enjoy Heroic Assault.
A reminder though, as while Heroic Assault is specified as a standalone mode, you will still need the Gotham Knights base game in order to access it. Once you purchase Gotham Knights, you will be able to download and play Heroic Assault for free when it is available.