Gibson, 28, joined Tigers from Salford in 2013. He has scored 11 tries in 28 appearances for Cas, and is sad to be leaving.
“I just want to thank everyone at the club and the Tigers fans,” he said.
“I’ve really enjoyed my time here especially this year when I’ve played regularly.
“I would like to wish the club all the best for the future and I’m sure they will be up there in the top four competing for major trophies soon. Thanks again.”
Welsh international Michael Channing joined Tigers in 2013 too, initially on loan. He then came in on a permanent deal in 2014.
“I’ve absolutely loved my time at Castleford over the last two and a half years I’ve been here,” he said.
“Unfortunately in life not everything works out the way you want it to but I hold no grudges and am just honoured to have represented such a humble and welcoming town.
“One day I wish to return to the Jungle and pull on the Cas jersey again but for now I look forward to my future challenges.
“I want to thank everybody at Castleford from the coaching staff to the fans who are there for us week in week out.
“As a southerner coming up North I have definitely found a second home where I know I’ll always be welcome. Thank you again everybody and see you soon!”
The third player to leave, Brandon Moore, is an England Academy international who joined Cas from the Cumbrian Regional Academy in 2014.
He was awarded a full-time deal in 2015, and won the award for Under-19ds player of the year for this season.
“Winning the Under-19’s Player of the Year award makes me realise how far I have come as a player this year and I would like to thank Daryl and all the coaching staff at Cas for all their time and giving me the opportunity to play at this great club.
“Moving on to next year I’ll take a whole load more experience with me and I’d like to thank everyone at the club for a great two years. I’m leaving a better player than when I came and for that I am so grateful.”
Castleford coach Daryl Powell thanked each of the three for their work during their time with the Tigers.
“Ash Gibson has been a consummate professional for the past two seasons,” he said.
“His ability to play in the centre and on the wing means he has done a great job for us as a utility back.
“I’d like to wish him all the best in his future career and thank him for his service as we have grown as a club over the last two years.
“Michael Channing has been a pleasure to have at the club. His infectious personality is great and his ability has grown across his two years at the club.
“I would like to wish him all the best in his future career and thank ‘Channers’ for being a part of the clubs development over the last two years.
“Brandon Moore has played for one season for the club at Under 19’s level but has been training with us as a full-time player.
“His toughness, discipline and focus is second to none and I’m sure he will do a great job in the Championship next season for whichever club he chooses.
“It’s been great to have him at Castleford and we wish him good luck for his future.”
Castleford have also confirmed that their recruitment for the 2016 season is ongoing, and negotiations are currently taking place to bring more personnel in.