She was partially naked, wrapped in power cords and covered with a hairdryer, a lamp and other household items.
Andrew Baker, her on-and-off boyfriend of more than two years denies killing her in April 2017.
But it's alleged he did - sometime between April 19 and his 50th birthday four days later.
The passage of time between her death and her discovery means it was impossible to determine her cause of death, but prosecutor John Dickie told jurors in the opening of Baker's murder trial on Tuesday that he allegedly acted with murderous intent against her, or at least with an intention to cause her really serious injury.
Baker has pleaded not guilty.
Mr Dickie described Baker as jealous, insecure, possessive and controlling. He didn't like that Ms Gatt also had another on-and-off again relationship with Leona Rei-Paku, he said.
That relationship was also violent and turbulent. Both involved multiple calls to triple zero, which will be played to jurors.
Ms Gatt had fallen on hard times, battling mental health and drug issues, had children outside her care and was on a disability support pension.
It's alleged her murder, a month before her 40th birthday, was motivated by Baker's jealousy and unrequited love.
Ms Rei-Paku last saw Ms Gatt alive on April 19, 2017. The power was switched off manually to her Kensington townhouse on April 23, giving police a timeline for the alleged murder, Mr Dickie said.
The power was turned back on in August 2017, when it's admitted Baker and others went to Ms Gatt's home and saw the body in the bath.
Baker told a friend the body might be Ms Gatt's. Mr Dickie said Baker later invited Ms Rei-Paku to the house and that she had pulled back a blanket and saw a foot.
Baker allegedly said he and his friend were going to clean up the house and "throw the body out".
Defence barrister John Saunders cautioned jurors against judging Baker and his friends for being in a house with a body and not calling police.
Those people did not live a normal lifestyle, he said.
Many, if not all, had mental health issues and battle drug and alcohol abuse, he said, describing them as people who live "on the outskirts of our society".
"Andrew Baker did not murder Sarah Gatt. He is not responsible for her death," he said.
It wasn't until January 2018, eight months after Ms Gatt's alleged murder, that police discovered her remains while doing a welfare check.
They had left a calling card for her in December 2017 and when they returned on January 3 it was untouched.
Mr Saunders said there was no direct evidence Baker killed Ms Gatt, nor was there evidence about her cause of death.
"In those circumstances how can you exclude, as a reasonable possibility, that Sarah Gatt didn't die as a result of injuries she sustained in an accident, such as falling down the stairs," he said.
He also said Ms Rei-Paku couldn't be ruled out, noting that in May 2017 she posed as her girlfriend at Centrelink and tried unsuccessfully to divert payments to her own bank account.
Jurors will be shown crime scene videos and footage from inside the home later on Tuesday.
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