Gannon made two appearances on loan for York in 2021 from Warrington Wolves before the forward joined Leigh ahead of the 2023 season.
The 21-year-old has yet make his debut for Leigh but last year represented AB Sundecks 1895 Cup champions Halifax Panthers and Betfred Championship promotion winners London Broncos.
Gannon, a former England Academy international, has also previously represented Rochdale Hornets, Bradford Bulls and Swinton Lions.
Knights Head Coach Andrew Henderson, who is proudly sponsored by Haxby Sports Bar, said: "I'm really pleased to be able to bring Jacob here on loan. Jacob has been here before, so he's got familiarity with our environment, and I've briefly worked with before at Warrington, so I've got familiarity with him personally too.
"We needed to get some more bodies in, with our current injuries and suspensions.
"But anyone that we bring in has to be of the right quality and the right personality to fit into our group and we believe that Jacob ticks all of those boxes.
"Jacob can play through the middle or on an edge, so he brings that versatility to our forward pack and he will certainly challenge our current forwards for a place in the side."
Gannon is eligible to feature for York this Sunday against Wakefield Trinity in the Betfred Championship (3pm).
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