Wales' all-time top try scorer has been tracked by Toulouse in recent weeks, and the deal was finally sealed when the European champions offered the Warriors a substantial cash payout and a pre-season friendly in August.
Thomas had two years left on his contract and the compensation package will go some way to appease Warriors chief executive Leighton Samuel who had become resigned to losing his prized asset.
Thomas, who has been on international fullback duty for much of the season has donned a Warriors jersey on 10 occasions this season.
He has played just four times in the Celtic League but has scored three tries - a hat-trick against Cardiff Blues on December 27.
Thomas played in all six Heineken Cup Pool matches, and scored in both games against Calvisano.
The Warriors have also ensured they have first refusal on 29-year-old Thomas, should he wish to return home at any time.
'We have accepted the offer made which, when including the game, will actually exceed our original demands for compensation,' said Samuel.
'We did everything we could to keep Gareth and we're very disappointed that he is going but he was determined to go and there seemed little point in standing in his way."
Thomas, who scored his 34th Test try for Wales in his 77th appearance for Wales during the 2004 Six Nations, is set to earn more than Â£400,000 over the three years.