The short video showed various scenes of Walker in a hospital.
"It is like a curse, it's quite scary, the unknown," he said in the video.
The feature-length event was first announced on February 15.
The trailer showed Walker with a medical dressing across his abdomen saying, "I just thank God every day that I'm alive."
The Herald understands Walker has had his stomach removed.
Last year, Walker discovered he has a rare cancer-causing gene mutation which runs in his family. The aggressive CDH1 mutation was responsible for his mother's breast cancer and reportedly for the deaths of at least 25 of his family members.
It's estimated 70 per cent of those who have the mutation will develop stomach cancer by age 40, so stomach removal is often the recommended treatment.
Walker's cousin recently had his stomach removed, and Walker paid tribute to him on social media earlier this month writing; "Proud of my lil cousin ... it's only been weeks since getting his stomach out from having cancer and now he's out and about with us all."
In February the 27-year-old posted a heartfelt message to his fans on Instagram.
"The past nine months have been a life changing journey full of ups and downs to say the least.
"As an artist I like to make every situation a creative experience no matter how sh*t it is. There is always much beauty to be found in heartache, loss, pain, trials and tribulations and even sickness.''
