"There's definitely a lot of pressure," he said in an interview at iSquare in Tsim Sha Tsui. Fans of the six-book series by Cassandra Clare already have strong mental images of Gao's character - Magnus Bane, the centuries-old warlock who's the guardian of the main character, Clary Fray - and how he should look on the big screen.
The Mortal Instruments series is about a group of people named shadowhunters, who get together to fight demons. Bane, a few hundred years old, is half-human, half-devil and possesses awesome powers.
He helps Clary's mother, an ex-shadowhunter, to erase her 15-year-old daughter's memory as she doesn't want Clary to develop the "sight" - the ability to see demons.
Although a loving character, Bane is also someone who likes to hang out with people while he enjoys being the centre of attention. That makes him less of a good caretaker than he appears to be.
"He's just out there. He likes to have fun. He throws a lot of parties. He wears crazy make-up and dresses crazy and has glittering nail polish. He's a pure rock star," Gao says. "But aside from being a party animal, he's still a very sentimental person. He treats Clary like his own daughter and is very caring."
But Bane's been alive for centuries, and deep inside his youthful looks, he is old and wise.
"He's been through it all and he's seen everything because he has lived for so long. Everyone that he fell in love with passed away in front of him. He's a very difficult guy to understand, having a lot of things inside his head," Gao says. "It must be tough to be this guy. I can't imagine never dying and never getting old. It's good in a way, but it's also frustrating. I can understand a little bit about his deepest emotions, and there's a lot to take in."
To get a better grasp of the challenging role of bringing Bane to life on screen, Gao said he talked to Clare, Dutch director Harald Zwart, and the producers.
Gao was in Shanghai, working on another film, when he heard about the auditions for The Mortal Instruments in the United States. He saw the possibilities of this new teen franchise and sent his casting tapes to the producers. Step by step, he was hired.
"I'm just grateful to be involved in such a wonderful project with so many beautiful cast members, directors and producers," the star says.
It seems the high regard is mutual. On her website, Clare had plenty of praise for Gao. She wrote: "Godfrey is, in addition to being all gorgeous and stuff, super-talented (his audition was awesome). He loves the books, and is, like Magnus, [very] tall." Imagine an imposing figure, 1.93 metres tall, with yellow and green eyes.
Gao urges fans to read the books first before watching the film, which will be released in Hong Kong on August 22. But even if you haven't had a chance to read the books by then, he adds: "The movie itself is going to be a great film, and you can understand the whole story behind it."