According to a statement from Selangor, the 33-year-old footballer has undergone surgery to remove the tumour and is being monitored by a medical team.
"Despite being diagnosed with the condition, he remains optimistic and in fact, stronger than ever. We hope everyone will pray for his health and provide him with some privacy during his recovery," the club said.
"Our captain is determined to recover and eager to come back and be with the team as soon as possible," it added.
The Australian-born midfielder also took to social media to express his desire to captain the Red Giants again once he wins his battle against the disease.
"Cancer won't define me. It's only a short scenic route that I will navigate through and get back to my blessed path that I was living prior to this," he posted on his Instagram account.
"The world has been crazy the past couple of years. Let's look out for each other and always search for the positives in every situation! My motivation is to fight the big 'C' and get back out there leading the boys very soon," Gan wrote.
Gan's first foray into Southeast Asia was with Sabah in 2011 and he has since then represented Kelantan, Perak and Selangor in the Malaysian first division.
The midfielder made his international debut for Malaysia in 2016 and was part of Tan Cheng Hoe's Harimau Malaya squad that competed in the 2022 FIFA World Cup qualifiers last month.
He has made 11 appearances for Selangor in the Super League this season but did not feature in the squad as his side drew 1-1 with Kuala Lumpur City on Saturday.