The Robins suffered relegation from League Two and boss Gary Johnson has taken action.
Trevor Carson, Jack Deaman, Omari Sterling-James, Matt Taylor, Lee Vaughan and Jamal Lawrence have been transfer-listed.
And Durrell Berry, Craig Braham-Barrett, Troy Brown, Danny Haynes, Mathieu Manset, Matt Richards and Matt Gould will not be offered new contracts.
Long-term absentees Asa Hall, Jordan Wynter and Eliot Richards – who are all contracted until the end of next season – will be assessed during pre-season and decisions will be made then.
Russell Milton has been confirmed as Johnson’s assistant manager for next season, with the club confirming negotiations are currently ongoing with the rest of the backroom staff.