Mr Perston collapsed and died at the scene behind a block of shops and flats off Gordon Rd.
In a two-hour video interview shown to the jury of eight men and four women, Gerrard said he was upstairs in a flat listening to a party that had become rowdier as the day went on.
Aware his brother was at the party, he headed below to tell people they'd been asked to leave by residents and should comply.
All in the gathering were intoxicated, and he focused on getting brother Logan to return to their flat. Gerrard said he was prepared to walk away because the crowd was not listening.
Mr Perston, later found to have had a significant blood-alcohol level, staggered towards him and introduced himself with a handshake.
Gerrard told the detective Mr Perston persisted in shaking his hand, while making comments such as: "You don't know who you're messing with".
The accused said he declined further handshaking as Mr Perston shouldered up to him and told him to "piss off".
Gerrard said he was walking away when Mr Perston tried to punch him from behind.
"I spun around and hit him once," Gerrard told the officer. "Not as hard as I could have."
He said he was immediately concerned about Mr Perston's condition, and told his mother to call the police and an ambulance.
He told Detective Martin James: "This guy was heavily intoxicated and unpredictable. I was only ever going to do it once, to get him off me.
"I just wanted to walk away and go home. I would have just turned the TV up and carried on with the night."
Gerrard did not give evidence in court and Mr Forster elected not to call witnesses for the defence.
While pleading not guilty to the charge of manslaughter, Gerrard concedes he delivered the punch which caused the death, but maintains that the punch was thrown in self-defence.
The judge was expected to sum up this morning before the jury retired to consider a verdict, awaited by family members of the victim and the accused.
Court security was boosted after concerns about earlier responses during the trial, which led to two men being ordered to leave the public gallery.