The forward spent last season on loan at York from Leigh Leopards, making 18 appearances and scoring two tries.
It was the 22-year old’s second spell at York, following on from two appearances for the club in 2021 on loan from Warrington Wolves.
A former England Academy International, Gannon joined Leigh at the start of 2023.
Since then, he's spent time on loan at Halifax Panthers, London Broncos and Rochdale Hornets, before moving on to the Knights in 2024.
Head coach, Mark Applegarth said: "I'm really happy that Jacob has agreed a deal to stay, coming in from Leigh.
"Jacob’s another player I coached on the England Pathways and I think he’s got a really bright future ahead of him.
"He did a fantastic job covering the back row for us last year when we had a few injuries. I know he sees himself as a middle so I think he’s really going to make a mark this year and make the shirt his own.
"I’m delighted he’s signed up and I’m really looking forward to working with him again."
Gannon is the sixth new signing for 2025, joining Paul McShane, Kieran Buchanan, James Farrar, Jude Ferreria and Jacques O’Neill.
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