Gallagher's ordeal was captured on video and a film crew from TV One reality show Code:1 recorded the aftermath in Paeroa. He was airlifted to Waikato Hospital by Auckland's Westpac Rescue Helicopter.
Viewers will see the drama unfold tomorrow night.
Following surgery to remove chips of bone from his hip socket, Gallagher was laid up at home in Gisborne for 3 months before returning to light duties as a farm stock manager.
"I was on the bull for five or six seconds before it threw me off," he said. "When I hit the ground I tried to get out of the way but it came around really quickly and hit me in the back with its head and ran me over.
"I was on the ground and couldn't move my leg so I knew something was badly wrong. At first I wasn't in pain because the adrenalin had kicked in, but once people got me to safety it was agony.
"The next thing I knew there were medics everywhere and I was facing a painful and bumpy ride to hospital in an ambulance. I was so grateful when the helicopter arrived and spared me the road trip. Those people were fantastic and I can't thank them enough."
Gallagher has been bull riding for seven years and despite his serious injuries hopes to be back competing in 18 months.
"It is a dangerous thing to do at times, but so is crossing the road," he added. "My wife isn't too happy about me getting back on a bull again but she knows I love the sport and I won't stop doing it just because I got hurt."
Code:1 screens tomorrow on TV One at 8pm.