When is the moment that Gerard realized Kimble was innocent?

I think it seals it for him when his associate stumbles upon that there was a call from Kimble's car phone to Sykes. He had probably gone over with the police where Kimble was the night of the murder and that Nichols had borrowed Kimble's car. Though he was suspicious obviously at the phone call to Sykes house and of Sykes himself. As he's leaving he says, "This guy's dirty."

Green Goblin is great! https://www.youtube.com/watch?v=t1L4ZuaVvaw

Not exactly sure when, but I will say this much, after the St. Patricks Day debacle and not being able to catch him, and when that Reporter reminded him of all the chances Richard took coming back to the city, and that Richard could be Innocent, Now that should have really registered with all Chicago PD and detectives as well.

What Journalist or Reporter in the last 30 years would even wonder or ponder such a question?

I think the same as most people here, that he was having his doubts ever since they learned that Kimble came back to Chicago and is searching for a one-armed man. But I agree with the other post here, that what nailed it was the call that Kimble's cell phone made to Sykes. The evidence that convicted Kimble in the first place, said that he killed his wife himself. So why would he need to call Sykes?? At that moment Gerard mustve realized that Kimble didnt call Sykes, that someone else using his phone did.

Finding out about the phone call to Sykes may have sealed it for Gerard, but I think his first suspicion came much sooner than that. I think it started in tunnel, when Richard was pointing Gerard's gun at him. This man was supposed to be a cold, calculating murderer, who had just survived a bus and train crash, and was leading police on a high speed chase. And now he has gotten a gun away from a cop, and has him cornered, but the only thing on his mind is "I didn't kill my wife!". Then Kimball doesn't kill Gerard, or try to take him hostage to barter his way out of the tunnel, but just walks away. This shows a different mindset to the normal criminals Gerard has dealt with.

My feeling is Sam "Big Dog" Gerard was skeptical from the moment he first met with Detectives Kelly and Rosetti. He's incredulous at their motive for Richard killing Helen: "What do you mean he did it for the money? He's a Doctor; he's already rich!". Gerard re-interviews everyone in their files, and by the end he is having a hard time reconciling the Dr. Richard Kimble his colleagues are describing with the man who supposedly killed his wife in such a brutal manner.

At Cook County Hospital, his doubts about the CPD investigation intensify when he and Cosmo realize that the prosthetic clinic is the reason Richard was "hanging around a trauma ward acting like Mother Theresa". The fact that Richard's reason for being in Chicago appears to be that he's hunting down the one-armed man lends credence to his story. If he's guilty, why would he be after a man that doesn't exist? Richard then leads Gerard to Frederick Sykes, and all he can say after questioning him is "Cosmo, this guy's dirty!".

At this point, Gerard is convinced there's more to this case than meets the eye, and begins to run a parallel investigation to Richard's. Gerard learns that Richard was looking into tissue samples relating to a drug trial study that he was involved with, along with Charles Nichols and Alexander Lentz. This means he just caught Nichols in the lie that he didn't even know Lentz. Additionally, The pharmaceutical company behind the trial is Devlin-MacGregor, which just happens to be the company that Sykes is employed with as Head of Security. He also discovers that Lentz was killed by a hit & run driver while Richard was in prison.

While all this information is beginning to paint a compelling picture, it doesn't actually go against the "husband kills wife" scenario the CPD is pushing. Gerard needs something irrefutable; a cold hard fact...which is why this is the perfect time to receive the info regarding the phone call from Richard's car phone to Sykes the night of the murder. Phone records also indicate if a call connected so this proves that Sykes was lying about being out of town that night.

Gerard begins to look through his paperwork for the original arrest report. Why? That report is fresh enough in his mind that he knew right away something was off about the timing of the phone call, but just didn't quite what know it was. The timeline indicated the approximate time Nichols returned the keys to Richard, which confirms to Gerard that the call to Sykes occurred prior to this exchange. This proves that Nichols contacted Sykes the night of the murder while he was in possession of the keys to the Kimble home. No forcible entry.

This is the evidence Gerard needed and the moment that he is convinced beyond any doubt that Richard Kimble is innocent.

As others pointed out, the crucial piece was when the phone records showed Kimble called Sikes from his car phone. They already had it on record that Nichols had the car at that time so there was NO chance Kimble could have made the call.