From the IMDb FAQ segment about this sentence:
A definitive answer is impossible. Christopher Nolan, when asked about an answer during a Blu-Ray Live Event, responded "I will answer this question one day. But not today."
However, I think this is the most reasonable explanation:
Detective Wuertz. Killed in the bar.
Detective Anna Ramirez. While Two-Face lets her live, he knocks her out. She would have been missing. Gordon, knowing she's the one who drove Rachel, might have assumed she was dead. Or, less likely, Dent inadvertently killed her - although he intends to let her live, she could have been severely injured (e.g., head striking pavement) after being knocked out by Dent, and subsequently died.
Maroni's Driver. Shot by Two-Face while driving along the scrapyard.
Salvatore Maroni. [...] Two-Face puts his seat belt on and shoots the driver, causing him to crash and flip the car. While Two-Face walks away from the crash unscathed, we never find out what happened to Maroni. We can assume he was either killed in the crash because he wasn't wearing his seat belt or left Gotham and went into hiding afterwards; either way, he isn't in The Dark Knight Rises.
Harvey Dent. Gordon could have meant Dent himself, because in The Dark Knight Rises the public is under the impression that Batman murdered Harvey Dent in cold blood. It is never mentioned who else that Batman had supposedly killed.