Officers dropped the 50-year-old homeless man near where he would later be found dead, but did not record the interaction, nor report it to a coroner until two years later.
Police also failed to notify Mr Gaskill's family about his death until four days after his body was found, on April 15 2022, and destroyed his phone after he died.
When he was found by a walker, on sand dunes at Ocean Grove beach, southwest of Melbourne, Mr Gaskill's body was too severely decomposed for a cause of death to be ascertained.
He had suffered a mental and physical health decline in the years before his death, including issues with alcohol and drugs, and was sleeping rough.
Police handed evidence and items discovered near his remains back to his family.
This included his backpack, laptop and personal papers, which the family then returned to police to re-open the investigation.
Coroner Ingrid Giles said these were among "significant deficiencies" in the police investigation after Mr Gaskill's death, as she handed down her findings on Friday following an inquest.
"There were significant failures in the initial police investigations of Simon's death that led to multiple coronial investigators being appointed, along with the involvement of Professional Standards Command and the Independent Broad-based Anti-Corruption Commission," she said.
The coroner took the "unusual" step of ordering the chief commissioner of Victoria Police to pay legal costs for Mr Gaskill's sister, Amanda, who was in court on Friday.
While she did not find police actions caused the death Ms Giles said their "complete initial failure" to conduct inquiries, investigate his death when it first occurred and report their interactions before he died had been unreasonable.
"Ms Gaskill was instrumental in providing the evidence that led to reopening of the investigation, and drawing the court to the deficiencies, that ultimately resulted in ... two internal reviews of the investigation."
She said Mr Gaskill was also failed by Barwon Health, after he was discharged from hospital in December 2021 without making note of his homeless status, nor recommending outpatient appointments.
"I consider that there should be clear processes to accommodate the support needs of vulnerable patients like Simon in these circumstances moving forward," Ms Giles said
The coroner made 10 recommendations to the chief commissioner, Barwon Health and Victorian government, including that police review their policies and guidelines in managing seized property.
She said uniform police members needed to be reminded of their requirements in recording and reporting on operational duties, including properly documenting incidents, and turning on their body-worn cameras.
Ms Giles recommended Victoria Police's manual require officers to check its databases for interactions with a person after they are found deceased when reporting the death to the coroner.
She further called for Barwon Health to review its processes with outpatients and better train staff in providing care to patients experiencing or at risk of homelessness.
Outside court, Amanda Gaskill said she hoped the findings would help the families of vulnerable people to ensure police's failures are not repeated.
"We are very grateful that after four very long, distressing, and gruelling years, some dignity has been brought to Simon and to the circumstances surrounding his death, notwithstanding the initial failures of Victoria Police in respect of their early investigations," she said.
"Simon was a loved son, a loved brother and uncle, a gentle soul, a good mate, he was easy-going, and a lover of the surf and nature.
"As a family, we hope that in fighting for the truth and the correction of failures, we have honoured his memory."
Victoria Police expressed their condolences to the Gaskill family and said they were especially sorry they had to wait so long for answers.
"We will now take the time to fully consider the coroner's recommendations," a spokesperson said.
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