Most of the people who died are thought to have suffocated in thick toxic smoke after the man set fire to casino tables at casino and hotel complex Resorts World Manila, BBC News reports.
At least 36 bodies have been recovered so far after the incident, which occurred in the early hours of Friday morning (local time).
Police said the gunman, who is yet to be named, later killed himself by setting himself on fire.
Officials have insisted that the incident was a robbery attempt, and was not terrorism-related, despite an apparent claim by so-called Islamic State.
The man is thought to have entered the casino shortly after midnight and began firing an assault rifle, sparking widespread panic, although police told BBC News none of the victims had gunshot wounds.
The gunman also set gaming tables on fire and filled a backpack with gambling chips worth 113 million Philippine pesos (£1.7 million).
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 7am local time (11pm UK time).
Authorities have also told reporters that the man appeared to be "mentally disturbed".
More than 50 people have been sent to a hospital to be treated for injuries.
