The typhoon made landfall in mainland China on Thursday evening local time, prompting authorities to declare the highest-level disaster alert.
The typhoon first struck Taiwan on Wednesday evening, killing three people as it swept across the island.
Four counties and cities in Taiwan received more than 1,000 millimeters of rain in a 14-hour period ending Thursday afternoon.
The Philippines escaped a direct hit from Gaemi, but the storm intensified seasonal monsoon rains and caused widespread flooding in Metro Manila and its suburbs.
The MT Terra Nova, sailing under the Philippine flag, did not violate any rules for navigation in bad weather despite the heavy rain, Rear Admiral Balillo said.
The tanker was heading to the central Philippine city of Iloilo when it sank with 17 crew members on board.
One person died, but 16 others were successfully rescued, officials said. Authorities are investigating whether bad weather was a factor.
The Coast Guard is now in a “race against time” to stop the spill, which if allowed to proceed at all could be the largest spill in the nation’s history.
