Okay, let's rule out a few obvious one.
(1.)
There was the series Gotham Central.
(2.)
There are also three four-issue mini-series from the 1990s collected in the Batman: Gordon of Gotham tpb.
* BATMAN: GORDON OF GOTHAM #1-4
* BATMAN: GCPD #1-4
* BATMAN: GORDON'S LAW #1-4
(3.)
More recently, we also had the Gotham by Midnight series, focusing on supernatural threats to the city.
Any other good stories focusing primarily on members of the Gotham City Police Department?