Following an itinerant spell in England, the defender returned to Scotland this summer to sign a two-year contract with Hibernian.
Born in Edinburgh, Gray moved to Old Trafford in 2004 at the age of 16 and made one first-team appearance. The time he spent at United continues to have an impression on him.
"I was very fortunate to get that education at United," said Gray, whose club are expected to appoint Everton coach Andy Holden to the backroom team. "It stands you in good stead on and off the pitch. It was different class."
Hearts also hope to profit from the experience of John Robertson, who is to rejoin the club as under-17s coach.