Gregg was born on a dairy farm in Campbell Town, a remote rural district in southern Tasmania.
In his early years he milked cows and ploughed fields for neighbouring farms, and was interested in Aussie Rules football, cricket and rowing.
Gregg auditioned for NIDA at the age of 18, and started studying with its first intake of students in 1959, alongside Robyn Nevin, Elspeth Ballantyne, Elaine Cusack, Teddy Hodgeman and Peter Couchman.
[2] Other television appearances included: Grass Roots, Special Branch, Armchair Thriller, Bodyline, Heatwave, Bootleg, Captain James Cook, Done Away with It, and was a featured player in Frederic Raphael's semi-autobiographical miniseries The Glittering Prizes for the BBC.
He had four children, Sophie Gregg (who is also an actor), Alice, Ben and Sam, and eight grandchildren, as well as two stepsons and step-grandchildren through his marriage to Jane.