Resorts World Manila was put on lockdown after the incident, which happened shortly after midnight local time (16:00 GMT).
Swat teams were deployed to the scene, and TV footage showed people fleeing the hotel in panic.
The Philippines police chief said they did not believe it was a terror attack.
Ronald dela Rosa said a lone gunman had entered the gaming area at Resorts World and set some tables on fire.
Gunshots were fired, he said, but were not aimed at people in the room.