It's bad timing. It would have worked better the first time Brook and Gunko interacted, when Imu still wasn't at Elbaf personally.
I think the issue with people discussing the timing is if you look at how the recent chapters have been framed I really don’t think there was ever a good time to do it
If he did it when Brook and Shuri first interacted then the complaints arguably would’ve been worse
Brook and Gunko met for the first time in chapter 1146 and they didn’t actually interact until 1147. That’s only 5
chapters before the Harald/Rocks flashback began
So we’d go from Brook’s flashback -> to Rocks’ flashback with little to no downtime at all
Not to mention Brook’s connection with Gunko still wouldn’t be relevant… at least not right then
Now there’s also a chance you’re talking about when Brook clashed with Gunko
after the Rocks flashback. Well if that’s the case then the timing is honestly even worse
Brook clashes with ImuGunko in 1173 which is only
2 chapters after the end of the Rocks flashback. Finally getting out of the longest flashback in One Piece history before jumping right into Brook’s after only 2 chapters? The complaints would be endless
Narratively (assuming Gunko remains frozen until the fighting is finished) it would make more sense to have the flashback take place closer to the end of the arc
but that causes another problem. The end of the arc is supposed to give us time to take a breather + provide us with updates on the outside world
Who would really want a Brook/Gunko flashback at that point? Just think about the other things that we’d have to go over
Louis Arnot’s warning, Biblo’s true identity, the mystery of the Heaven World, what Gaban wanted to talk to the Strawhats about, Loki’s past with Shanks, the location of the final Road Poneglyph, Mosa’s identity, etc
Oda’s stuck between a rock and a hard place. In order for a Brook/Gunko flashback to work without messing up the pacing he’d have to completely alter the events/setup of the arc. Cause with the way he wrote it there just
isn’t a good place to slow the story down with a flashback