The club were last week informed that the lock intends to move to Edgeley Park next summer, after completing a fourth campaign as a Glasgow Warriors professional.
And Gray, who has 16 caps for his country, admitted it was a real wrench to leave his boyhood club behind.
"I'd like to put on record my thanks to Glasgow Warriors for the support they've given me in all aspects of my life as a professional rugby player," he said.
"Being a proud Glaswegian, it's been a massive honour to wear the blue and black jersey, and I'm looking forward to enjoying more special days in it before my time here ends.
"I was impressed by the efforts that the club and Scottish Rugby made to keep me in Glasgow, and it's without doubt the toughest decision I've ever had to make.
"This isn't about money: I've always been keen to test myself in England, and I feel this is a good opportunity to do so.
"It's a positive thing for all concerned to have clarity on the situation, and my full focus remains on helping Glasgow continue our excellent recent run."
Warriors chief executive, Kenny Baillie, told the club's official website: "We're clearly very disappointed that Richie has decided to move on at the end of the season.
"A concerted effort to retain his services has been made across many weeks by a significant number of people within Glasgow Warriors and Scottish Rugby.
"The package we presented to him was certainly substantial from both a playing and financial point of view, and Richie has informed us that this was an extremely difficult decision to make.
"We'll be sad to see him go, but it's important to emphasise that Glasgow Warriors are far from a one-man team, and Richie's departure will not affect our and Scottish Rugby's commitment to ensuring the club continues to grow on and off the field."