Most of the dead suffocated in thick toxic smoke after the man set fire to casino tables, officials told the BBC.
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.
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 a claim by so-called Islamic State (IS).
The group, which posted the claim on Friday via the messaging app Telegram, offered no further details.
The man, who has yet to be named, entered the casino shortly after midnight and began firing an assault rifle, sparking widespread panic.
“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).
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).