Gathercole swam for the West Wyalong Swimming Club and received coaching advice through the mail from
Forbes Carlile. Gathercole first came to prominence in the 1954 Australian Championships, when he won the first of his ten Australian Championships. He made his international debut at the
1956 Summer Olympics in Melbourne, contesting the 200-metre breaststroke, at the first Olympics in which the
butterfly stroke was separated from breaststroke swimming. He finished fourth in the final, just 0.1 of a second behind the bronze medalist
Charis Yunichev of the
Soviet Union. He was coached by
Forbes Carlile after moving to Sydney. Gathercole reached the peak of his swimming career in 1958 when he set the
world record for the 200-metre breaststroke at the
Tobruk Pool in Townsville, Queensland. He held this world record for over three years. In the
1958 British Empire and Commonwealth Games in Cardiff, Wales, he won the 220-yard breaststroke and combined with
John Monckton,
John Devitt and
Brian Wilkinson to claim the 4x110-yard medley relay. Gathercole's final appearance on the international arena as a swimmer was at the
1960 Summer Olympics in Rome, where he won his semifinal of the 200-metre breaststroke. However, in the final he only managed sixth, almost three seconds behind
Bill Mulliken of the United States. Gathercole then combined with
David Theile,
Neville Hayes and
Geoff Shipton to claim silver in the 4 × 100 m medley relay, again behind the Americans. Gathercole had at various times in his career held the world record in the 200-metre, and national records in the 110- and 220-yard breaststroke. ==Coaching career==