Josie Murray - the grandmother of the four-year-old who vanished from her South Australia homestead in September 2025 - was fined $10,500 after a hearing in Adelaide Magistrates Court on Friday.
Murray, 75, of Yunta, had pleaded guilty to one aggravated charge of possessing a sound moderator.
Defence lawyer Andy Ey told magistrate Roderick Jensen that he would "address the elephant in the room".
"It's been an extremely difficult time for my client. A grandchild went missing on the property late last year," he said.
"We see the media attention and number of police here - this has been a very, very difficult time for the family, trying to get through as best they can."
Gus Lamont was last seen by one of his grandmothers playing at the family's Oak Park Station on September 27.
On February 5, police declared the boy's disappearance a major crime and said someone living at the remote station was a suspect in the case and his likely death.
They confirmed his grandmothers, mother and younger brother were at the property at the time he disappeared but emphasised his parents were not suspects.
Police had executed a search warrant at Oak Park on January 15 over an "unrelated matter", prosecutor Tania Stevens told the court.
Inside a passcode-protected strongroom, they located the silencer and several firearms, including one that was modified to fit the silencer.
Murray had a previous conviction in 2010 for falling to securely store firearms, Ms Stevens said.
That offence arose as a result of a fugitive trespassing on the defendant's property and stealing an unsecured firearm.
"That person then went on to commit serious offending with the firearm before taking his own life," she said.
Mr Ey clarified that two non-working firearms were taken, while a third one was taken from Murray's auntie's property later that day, "which is the firearm that that trespasser used to take his own life".
"That was an extremely traumatic incident for the entire family, with my client's auntie being held hostage," he said.
Mr Ey urged the magistrate to reject the prosecutor's call for a jail term for the offence, which carried a maximum penalty of $75,000 or up to 10 years in jail.
"This isn't some James Bond movie where you've had a villain or an assassin walking around the corners taking out targets with little more than a whisper of a gun - that is not what the court is dealing with," he said.
Mr Jensen imposed a fine of $10,500 and disqualified Murray from holding a firearms licence for five years.
Gus Lamont's disappearance sparked intensive searches spanning almost 500sq km and involving hundreds of police and volunteers as well as aerial support and mounted units, which have failed to locate Gus or any items belonging to him.
