Most of the dead suffocated in thick toxic smoke after the man set fire to casino tables, officials told.
The gunman, whose nationality is not yet known, began shooting in the casino in Resorts World Manila in the early hours of Friday morning.
Police said he later killed himself by setting himself on fire.
The incident sparked a security lockdown. Authorities initially said there were no casualties, but appear to have only found the bodies while sweeping the complex after the gunman's body was recovered.
Officials insisted that the incident was a robbery attempt, and was not terrorism-related despite an apparent claim by the so-called Islamic State (IS) group.
How did the attack unfold?
The man, who has yet to be named, entered the casino shortly after midnight and began firing an assault rifle, sparking widespread panic.
He fired into the air, not at people, but it caused fear, Southern Police District Superintendent Tomas Apolinario told the BBC.
"We have some injured people and we have some dead as well, but none of them have gunshot wounds," he said.
The gunman also set gaming tables on fire and filled a backpack with gambling chips worth 113m Philippine pesos (£1.7m, $2.3m).
Smoke was seen billowing out of the complex from a window early on Friday
The man later fled to the hotel section of the complex, leaving behind the backpack, and entered a hotel room.
"He lay down on the bed, covered himself with a thick blanket, apparently poured petrol on the blanket and burned himself," national police chief Ronald Dela Rosa said on Friday.
The shooting sparked a security lockdown and a manhunt in the complex, which ended when the gunman's body was found.
Resorts World Manila said in a statement that the man also shot himself after setting himself on fire, and that he was found dead around 07:00 local time (23:00 Thursday GMT).
Who was the gunman?
Police released CCTV images of the man, showing him dressed in black and carrying his rifle.
There has been some speculation on the part of local media and some officials that the gunman could have been a foreigner on the basis that he spoke English well. Early reports said he appeared to resemble a Caucasian.
The man was seen on CCTV in a stairwell within the casino complex
But Superintendent Apolinario told the BBC the police had not confirmed his nationality.
Authorities have also told reporters that the man appeared to be "mentally disturbed". Resorts World Manila has called the casino shooting a "cowardly act of a deranged mind".
Police say the gunman was acting alone, unconnected to any Islamist group. Over the last week militants allied to IS have been engaged in street battles in the far south of the Philippines.
How did so many people die?
Police said the suspect poured petrol over the furniture and rugs, causing them to catch fire immediately. Although the fire was not large, thick smoke caused the deaths.
Those who escaped the casino have said they saw smoke filling the complex.
One woman told AFP that she had received a call from her daughter as the attack happened. "Our daughter called us past midnight saying she was in the VIP section of the casino and there was smoke and they were suffocating," Gil Yongco said.
Resorts World Manila chief operating officer Stephen Reilly has told reporters that fire safety equipment worked, and that there was "proper ventilation".
More than 50 people have been sent to a hospital to be treated for injuries, including fractures.