The 29-year-old DR Congo centre-back made 252 appearances in two spells, having initially joined Posh in 2008.
Fellow defenders Shaun Brisley and Andrew Fox and strikers Souleymane Coulibaly, Kyle Vassell and Jack Friend have also left the club.
South Africa left-back Kgosi Ntlhe is available for a move after rejecting two contract offers.
Director of football Barry Fry said: "I would especially like to thank Gabi Zakuani, who has been a magnificent servant both on and off the pitch to Peterborough United.
"I would like to wish him every success in furthering his career. I am sure he will have plenty of options."
Ben Alnwick, Harry Anderson, Dion-Curtis Henry, Erhun Oztumer, Jack Payne, Ricardo Santos, Michael Smith and Jon Taylor are all entering the final year of their contracts.
Posh's club policy is to transfer-list players in those circumstances, but, if Graham Westley's replacement as manager wants any or all of the eight players to have an extension, then negotiations will take place.
League One side Peterborough United have released six players, including defender Gabriel Zakuani.