The 25-year-old black prisoner died from spinal injuries last year after being shackled without a seat belt in a police van, sparking days of racial unrest in the city.
Closing arguments in the trial of Police Officer Edward Nero concluded Thursday. He is the second officer to be tried.
Judge Barry Williams said he will deliver his verdict in the bench trial on Monday -- more than a year after Gray's death on April 19, 2015, became a symbol of the black community's distrust of police and triggered days of violent protests. Three of the officers charged are white, three are black.
Nero is charged with second-degree intentional assault, two counts of misconduct in office and reckless endangerment in connection with Gray's arrest and death in police custody.
On Thursday, Williams aggressively questioned prosecutors about their assertion that Nero and other officers assaulted Gray by touching him without reasonable suspicion or probable cause.
Prosecutor Janice Bledsoe argued that searching and handcuffing Gray constituted an assault.
"You are saying an arrest without probable cause is a misconduct in office charge -- is a crime?" the judge asked. "So you say if you arrest someone without probable cause, it's a crime?"
"Yes," Bledsoe responded.
Williams at one point said: "If you touch someone, it could be assault, it could be a hug."
Michael Schatzow, chief deputy state's attorney, later told the judge that "not every arrest that occurs without probable cause is a crime." But he added that arrests in which the actions of the officer "are not objectively reasonable" are criminal.
The prosecutor said Nero and his partner turned a routine "Terry stop" -- a brief detention based on reasonable suspicion of a crime -- into a full-blown arrest requiring probable cause.
In her closing, Bledsoe cited statements in which Nero and his partner both used the word "we" to describe putting Gray on the ground and handcuffing him.
Nero's partner, Officer Garrett Miller, testified that he detained Gray himself.
The state compelled Miller -- who's awaiting trial -- to take the stand against Nero with immunity, meaning his testimony cannot be used to incriminate him at his trial.
Bledsoe also told the court that Nero ignored a general order for officers to secure suspects in vans with seat belts.
Defense attorney Marc Zayon said in his closing argument that Nero acted legally and that handcuffing someone who ran from the police was "the right thing to do."
Zayon said Nero and Miller used the term "we" because they considered themselves a team.
The only time Nero touched Gray was when the suspect asked for his inhaler, Zayon said.
The defense argued there was no proof Nero had read an email about the seat belt order, and that the state had failed to show he had a duty to secure Gray. Zayon said it was the responsibility of the van driver to put a seat belt on the prisoner.
Nero was one of three bike officers involved in the initial police encounter with Gray that day in April 2015.
Prosecutors have said that Gray complained of having trouble breathing and asked for medical help as he was driven in the van. When he arrived at a police substation, he was unconscious.
A week later, Gray died at a hospital from his spinal injury.
Four officers have yet to stand trial -- Miller, Lt. Brian Rice, Sgt. Alicia White and Officer Caesar Goodson Jr.. The trial for Goodson, the van driver, will start June 6.
The case against William Porter, the first officer to go on trial, ended in a mistrial in December after jurors couldn't agree on a verdict.