Gary John Gilmour was born 26 June 1951 in the Newcastle suburb of Waratah. He attended Waratah Primary School and Newcastle Boys High School.

He was awarded two "Blues" by the New South Wales Combined High Schools Sports Association: in 1967 (Baseball) and 1969 (Cricket).

Gilmour was selected to play for Newcastle while still a teenager. He was only 16 when picked for Northern New South Wales against New Zealand and took 5–70 for Newcastle against Metropolitan when only 17.

He was picked in the Australian Schoolboys team to tour the West Indies in 1969–70.

In 1970–71 he was selected in the State Colts team.

1971–72: First Class Debut

Gilmore was selected in the New South Wales team to play South Australia in January 1972. He scored 40 in the first innings and 122 in the second. He also took 2–27 and 0–40.

Gilmour scored a duck in his next game, against WA, but took four wickets with the ball. His third match, against South Australia, saw him take 4–69 in South Australia's second innings.

1972–73

Gilmour began the next summer well taking seven wickets against WA including his first five-for, 5–65, and scoring 72 runs. He struggled against South Australia, scored 53 playing against the touring Pakistan side for an invitational Tasmanian side took five wickets against Victoria and five wickets plus an innings of 73 against Queensland. People began to discuss him as an international prospect.

1973–74

The following season, he had early good form including seven wickets against WA, six wickets and a score of 59 against SA, four wickets against SA, and five against WA.

A good all round performance for New South Wales against the touring New Zealand team helped his cause considerably, taking seven wickets and making a score of 54. He was picked in the Australian side for the first test.

Before that game he almost took NSW to victory against Victoria, scoring 72 not out in an unsuccessful run chase and nabbing four wickets.

Test Debut

Gilmour had an excellent debut. Australia batted first and scored 8–462; Gilmour scored 52 not out off 58 balls. New Zealand were dismissed for 237 with Gilmour taking 4–75. He only bowled three overs in the second innings as Australia's spinners took the bulk of the wickets; New Zealand only made 200.