The 29-year-old - who was signed by then Sunderland manager Roy Keane for ?9million in 2007 - has had an injury-plagued time with the club and made just one appearance for them last season.
Gordon - capped 40 times and included in Craig Levein's recent squad for an upcoming friendly with the United States - played just 95 times for the 'Black Cats' which saw him break his arm twice among other injuries.
He fell down the pecking order as Belgian goalkeeper Simon Mignolet made the first choice spot his own while Keiren Westwood leapfrogged him to being second choice.
O'Neill also sent Danish striker Nicklas Bendtner, former England fullback Wayne Bridge and Greek veteran Sotirios Kyrgiakos back to Arsenal, Manchester City and Bundesliga outfit VfL Wolfsburg after loan spells.
Source: AFP
Source: AFP