I was hoping we'd get to play some Gotham Knights this year, but alas it looks like we're going to have to wait. The official Gotham Knights twitter account tweeted out that they are going to be delaying the game until next year.
There's no details other than they want to make the game as best as possible and that's always a good reason to delay. If it's not ready, it's not ready. Look at the latest big example, Cyberpunk 2077. That game needed another year or so and now the consensus was it was released way too early.
You only get one chance to make a first impression and I'd rather the developers take the time to do it right than try to rush it out the door.