The hooker started his career at Franklin's Gardens, making 20 appearances, before joining Harlequins where he has won the Premiership title in 2012 and represented England in 2014.
With competition for the number two shirt high at The Stoop, Gray has been signed by the Saints as injury cover until October.
Boyd has been looking for a quality alternative due to a number of his own hooking options carrying injuries.
He believes the 30-year-old can provide the perfect solution, especially with youngsters Reece Marshall and Samson Ma'asi still learning the game.
"We're delighted to have secured a player of Joe's experience and quality for the next three months," said Saints' director of rugby Boyd.
"A number of our hookers are carrying niggles heading into the back end of preseason, and we want to make sure they are back to full fitness before they return with a long season ahead. We have three important preseason matches in the coming weeks, so it's important to make sure we are covered.
"Joe has a proven track record in the Premiership, and I'm sure our younger hookers like Reece and Samson will also relish the opportunity to learn from someone new."