Gibb, 53, died on Sunday after being admitted to a Miami hospital for an operation on his intestines. He suffered a heart attack before undergoing surgery.
The autopsy report, released on Thursday, said Gibb suffered from a condition called a volvulus.
It was unclear whether he had been aware of it.
His brothers Barry and Robin Gibb had been talking to officials at the hospital where their brother died, after expressing their concern over his treatment.
Speaking to the BBC at the weekend, they questioned whether doctors at the Mount Sinai Medical Center had been right to operate on him after he suffered cardiac arrest.
The autopsy report described a volvulus as a twisting of the intestine that restricts blood flow and obstructs the bowel.
Doctors said the complaint sometimes caused severe pain but sometimes produced no symptoms.
The singer's private funeral was held in Miami on Wednesday.
The ceremony - held at Riverside Gordon Memorial Chapel on Miami Beach - was followed by a cremation service.
The only stars reported to have attended the service were Michael Jackson and Scottish singer Lulu - who was married to Gibb for four years from 1969.
The Gibb brothers, who grew up in Manchester, have been performing together since the late 1950s.
They made almost 30 albums and sold more than 110 million records in their career.
Their 1977 soundtrack to the film Saturday Night Fever sold more than 40 million copies worldwide, making it the best-selling movie soundtrack ever.
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