Is that what they're saying? I do like Batman's line.
Yep;
Batman - "Who's the mustache with the broken nose?"
Gordon - "That's Kenzie, Narcotics"
Batman - "He's one of the guys I got into it with at the Iceberg Lounge"
Gordon - "You saying Kenzie moonlights for The Penguin?!"
Batman - "No, he moonlights as a cop"
He's basically telling Gordon that Kenzie is corrupt and a rat working for The Penguin, having previously beat him up at the Iceberg Lounge. Now, Kenzie obviously knows that, by now seeing him working at GCPD, the Batman has figured him out. Seeing the Batman speaking to Gordon, Kenzie's probably thinking that the Batman is exposing his cover to Gordon, and Kenzie will now likely want Gordon silenced/killed.
By punching Gordon, Batman is eliminating any thoughts in Kenzie's mind of any kind of alliance between Gordon and the Batman. In fact, probably now making Kenzie believe that the Batman is an enemy to Gordon, and therefore Kenzie's secret probably wasn't exposed. Gordon is no longer a loose end that Kenzie needs to "deal with".
By punching Gordon, Batman has bought Gordon time to rat out the corruption in GCPD, and likely saved Gordon's life, as he now won't be a target for Kenzie and other corrupt cops in GCPD.