According to the British retailer the game collection, the Knights of Gotham page shows that it will be available on July 31, 2021. But July 31 is Saturday, so it may just be a place holder for the July release.
Last December, the official twitter of Knight Gotham released a Christmas greeting map, which also showed information about July. The poster on this congratulatory picture shows that the circus will enter Gotham from July 16 to July 21. So at that time, many fans speculated that knight Gotham would be on sale in this period of time.
Both the official Christmas map and the retail store page point to July 2021, which makes fans increasingly speculate that this is the release month of the game. Before the official announcement, however, it was only speculation.