The 25-year-old, who came through the FOSROC Scottish Rugby Academy, has played 108 times and scored 10 tries for the club to date.
He was part of the squad which won the Guinness PRO12 trophy in 2015.
While at Scotstoun, Gray made his debut for Scotland against South Africa in 2013 and has since gone on to play 55 times for his country.
Gray said: "Leaving Glasgow will be tough. I love Glasgow and I'm so grateful to the club and so many people associated with the club for all they have done during my time here.
"I see the move not only as a chance to experience something new, but at the same time it's a chance for me to develop myself, not only as a player, but as a person.
"Without doubt this was one of the toughest decisions I've ever had to make - and I will be giving everything between now and the end of the season to ensure my time here finishes on a high."
Glasgow Warriors Assistant Coach, Kenny Murray, added: "I've been fortunate enough to coach Jonny for the entirety of his professional career to date, and it's been a privilege to watch him develop into the world-class rugby player he is today during his time in Glasgow.
"He's an example of what can be achieved with hard work and dedication and we're extremely proud of him.
"We'll obviously be sad to see him leave the club at the end of the season, but when he does we'll wish him well. I'm sure he'll be as determined as ever to end his time here on a high."