The cause was a tumour in his chest pushing against his heart and lungs. Other tumours were growing in his neck, lungs and kidneys.
He was told he could have just two weeks to live if he didn't start chemotherapy right away. The treatment lasted seven months and cost him his hair, 30kg in weight and sometimes his will to live.
''By the end of it I was a shadow of a man ... I looked like a 67-year-old man and I was only 29,'' he said.
The chemo also made him vulnerable to germs so he was afraid to meet people, go to the supermarket or return to the gym.
However, it also had the desired effect against the cancer. One month after treatment ended he started walking again, playing the odd round of golf — even that was a struggle at first — and going to the gym.
One of the things that helped him through his darkest days was the huge support he received from Northlanders, he said.
Both GJ and his wife, Sasha, had to put their jobs on hold so his gym instituted a fundraising swear jar, golf clubs held raffles, and mates organised a sponsored walk.
More than 130 people turned out despite heavy rain. ''When they had that walk in Kerikeri the chemo had really started taking its toll. I was getting down, struggling mentally and physically ... To see the community come out like that really gave me strength,'' Hill said.
''A lot of the people there didn't even know me but they came out anyway, that's the beauty of Northland.''
Hill is currently in Perth visiting family. On Monday he will fly to Las Vegas to compete in the WBFF (World of Beauty, Fitness and Fashion) bodybuilding world champs in Las Vegas.
Even Hill seems surprised by how quickly his strength, and his physique, returned.
''But I know how to train and I did all the right things,'' he said.
He will need ongoing checkups but his future prospects are positive.
''I believe so and I just have to keep believing it. I'm alive and kicking and I'm waking up every day. There were times when I was having treatment I was scared to go to sleep. I'd stay up until 3am, 4am because I was afraid I wouldn't wake up. So to be where I am now is really special.''
By showing people how he has transformed himself, Hill hopes he can give hope to others going through hard times. But he also wants to share his gratitude.
''I'm absolutely thankful to everyone who had my back since day one, and that includes people who don't know me. I just love that.''