Gray heading to Toulouse
The big lock will leave Castres for their Top 14 rivals at the end of the season after two years at the Stade Pierre-Antoine.
Gray admitted it was a very difficult decision to leave Castre but insists he could not turn down the opportunity to join Toulouse, who won Europe's premier cup competition in 1996, 2003, 2005 and most recently in 2010.
The former Glasgow forward said: "This was a very difficult decision to make, I have loved my time in Castres, the club and the supporters have made my time here very special and I will leave with very fond memories.
"However the opportunity to play for one of Europe's most successful clubs, steeped in history, is an incredible one for me. When I met with the club they described their ambition and I felt this was the right move for me, to challenge myself in what is a very strong squad.
"I still have seven months here at Castres, I will give everything I can to the club in hope of achieving top six and then who knows. My goal for now is to leave Castres on the best possible note, I will give everything I can to the club, the players and the coaches."
"It's a big signing which will strengthen our squad next season," club president Jean-Rene Bouscatel told a press conference in the southwestern French city.
Gray, 26, capped 51 times for Scotland, has played at Castres since 2013 following a year at English club Sale and four seasons with Glasgow Warriors.
Bouscatel was speaking during a press conference following Toulouse's return to their Stadium following a two-and-a-half year multi-million-euro renovation programme.
Toulouse will host Top 14 rivals Toulon on December 27 in their stadium, English club Saracens in the European Cup late January and Stade Francais in March.
"It's a huge relief and great joy to return to the Stadium," said Bouscatel, as Toulouse sit second in the Top 14 two points behind Clermont.
The Stade Municipal de Toulouse, which also hosts the city's football team, has boosted its seating capacity to 30,000 places as it will host games during the 2016 European football championship.
