Of those released are two of the Grand Final-winning team: former Leeds winger Leroy Rivett – who scored four tries for the Rhinos in the last Challenge Cup final at Wembley – and loose forward Dwayne Barker.
Also leaving is French centre Damien Couturier, hooker Andy Ellis and prop Gareth Price who finished the season on loan to Celtic Crusaders.
The club have also said that prop James Garmston will be allowed to go on a season-long loan at an LHF National League One club, while hooker Phil Joseph will be allowed to speak with other clubs.
Rovers chief executive Paul Larkin said: "The club would like to place on record their thanks for the contributions each of these players has made to Hull KR and wish them well for the future."
"After an injury-wrecked season at Leeds I am really looking forward to showing what a fully fit Mark O’Neill can do," he said.
"I am coming over to an outstanding club that’s professionally run, the facilities are great and they are a great bunch of blokes.
"I saw the final and I was impressed. It will be a tough year, as it is for any team that gets promoted.
"But it is a challenge and we have got a lot of young kids, who are very keen, enthusiastic and full of confidence."
Lakin added: "In Mark, we have signed a forward of the highest calibre and one who has played at the highest level all his career. His leadership skills and expertise will prove invaluable."
Gannon has signed a two-year deal at the club and is exempt from the overseas quota due to his British citizenship
"Jim is exactly the type of prop that Justin Morgan was looking for to complement our current front rowers and also the additional players we have identified," continued Lakin.
"He is a no-nonsense front rower with bags of Super League experience."