Mr Gourley, from Carrickfergus, was last seen in March 2009 in the Glengormley area.
The PSNI’s Major Investigation Team (MIT) said in September 2014 that Mr Gourley, from Castlemara in Carrickfergus, was last seen in the Glengormley area.
At the time of his disappearance he was described as “vulnerable”.
Mark was last seen in that vicinity around 9.30am on the morning of Saturday, March 7 2009.
He has had no contact with friends or family since then and police have been concerned for his safety.
Mark, who is considered by police to be vulnerable, is described as 5 feet 9 inches tall, of medium build, with fair cropped hair.
His right eyebrow is pierced, and he has tattoos on his left shoulder and left forearm, and a tattoo on his right arm bearing his Christian name.
Mark was last seen wearing a black waterproof jacket with a grey design on the shoulders, black Umbro tracksuit bottoms, black leather trainers and a blue Rangers Football Club woollen hat.
Police launched a murder inquiry in 2013 after saying it had “received new information”.
Detectives now believe he was murdered by the UDA in Carrickfergus.
His remains have never been found which has caused deep upset and stress for his families..