He worked with musicians including John Lennon, Ringo Starr, George Harrison, Mick Jagger, Ron Wood, Bill Wyman, Jimi Hendrix, B.

King, Bobby Goldsboro, the Animals, Stevie Wonder, Carole King, Frank Zappa, Sly and the Family Stone, The Velvet Underground, CSNY, Rod Stewart, Ravi Shankar, Keith Moon, Barbra Streisand, and Neil Diamond.

He also worked with producers such as Wes Farrell, Tom Wilson, Chas Chandler, Jack Douglas, Robert Margouleff, Phil Spector, and Bill Szymczyk.

9 of the 50 Greatest Moments at Madison Square Garden; he also did the remote concert recording starring James Brown for the Muhammad Ali/George Foreman fight, "Rumble in the Jungle", in Zaire in 1974.

Even if he had done nothing else in his life, Kellgren would be famous among musicians for a jam he produced in March 1975, a never-released song called "Too Many Cooks".

Present for the session were John Lennon, Stevie Wonder, Billy Preston, Mick Jagger, Al Wilson, Harry Nilsson, Jim Keltner, Ringo Starr, and Danny Kootch.

He single handedly was responsible for changing studios from what they were – fluorescent lights, white walls and hardwood floors – to the living rooms that they are today.

[5]"There are tales told by the campfire, where rock mythology is discussed, that say the atmosphere created in that first studio and the ones to follow was so close to a good home and a fine hotel that songs were written to immortalize that special state of mind ...

Stone later said in an interview that Gary, having worked all night, would very often be out cold on the couch, with Jimi still in the studio glaring through the haze at the speakers.

Almost two years after the launch of the New York Studio, Kellgren and Stone opened up the Los Angeles Record Plant.

[8] Guitarist Ronnie Wood wrote that Kellgren probably died from electric shock while trying to fix some underwater speakers in his pool, and that Gaines drowned trying to save him.

[10] Kellgren was survived by his wife Marta, their two children Devon and Mark, his sister Aleda, and his mother Crystal.