Gray, a member of Nottingham Forest's 1959 FA Cup-winning side and appointed player-manager of the Lions in 1963, won promotion to the old Third Division in his first full season and led the club into the old Division Two for the first time in 18 years the following season.
During those two years the Lions set a 59-game unbeaten run at The Den.
Former Millwall player Billy Neil said: "Billy was the best manager I played for. He was a winner. He taught me a lot and was so helpful to myself and the other younger players at the club.
"He was a delightful man to work for and a real gentleman. This is very sad news."
Source: PA
Source: PA