(Yes, I feel like Pennyworth / Gotham, Superman & Lois, Gotham Knights and even Krypton are adjacent to the Arrowverse in some way via Hypertime or malleable histories that have since been changed by a Crisis event or ultimately unseen backstory and dependent on the audience piecing it together themselves. "Canon-adjacent", but more complicated if DC were to explain it but in the end chose not to. BUT overall not "Arrowverse" since they are far-and-apart, standalone from that narrative; with Constantine being the sole standout of the adjacent standalone shows that became a part of said narrative.)
^ unrelated to my post explaining "why 2015?" above, just speculation I didn't want to attach to it.
Ok so there's no real reason close to confirming that Gotham Knights takes place in 2015?
I mean, I've tried to find some date and the only thing I found was in the pilot episode where it says it started in January of some year.
On the wiki they put Bruce Wayne's DOB in 1965 but I couldn't find where they got that information from.
Now it turns out that Harvey Dent pretends to be 56 during the events of the series.
But using that age could fit in many places, the one you mentioned that means that it's his age in 2023 and that we should go back to 2015 to fit with Misha Collins' age during filming.
It could also be interpreted that it's in January 2031 assuming that Harvey and Misha were born in the same month and year.
or just Harvey is 56 in the year of the series or in 2023 and it's just an indicator to know when he could have been born, which calculating it, gives us that he was born in 1967 (close to 1965 that the wiki mentions for Bruce Wayne's DOB)
So we have 2015, 2023 and 2031.
I will continue my search, thanks for your reasoning linked to the Arrowverse, I would like to understand it a little more if only there was a real connection between the arrowverse and Gotham Knights, be it someone from the cast or confirmation that it is part of the same multiverse or whatever.
I don't know if it helps but I remember that during production, the props manager posted an initial Batman mask on his instagram and when he was asked if this series shared continuity with Gotham, he said no, that it is supposed to be connected to Robert Pattinson's Batman.
Then in the pilot episode there is a Batman poster that makes reference to a real one of the batman with the red background even. After that, no more things related to it were released, but there were also rumors that it apparently existed in the same universe as S&L, or at least that's what a user on YT said, who swore to have spoken to David Ramsey about this and Justice U in mid-2022.
I'll keep looking for a real year (also, Arrowverse Batman as far as we know disappeared, he wasn't killed, we don't even know if by the Court of Owls in the Arrowverse, I don't think they are comparable situations to relate them temporally with each other)