In 1909 Gillan joined the Sudan Political Service, but returned on leave in 1911 and as a member of Leander Club and was in the crew that won the Grand Challenge Cup at Henley in 1911. He was home on leave again in 1912 and was member of the British eight which won the gold medal rowing at the 1912 Summer Olympics in Stockholm. He was in the UK again in 1917 when he married Margaret Douglas Ord Mackenzie at Holy Trinity Church, Brompton.[4]
Gillan served in the Sudan for thirty years and became Civil Secretary in 1934. He was appointed a CMG in 1935 and a KBE in 1939.[5] After World War II, Gillan headed the Empire Division of the British Council and played a major part in the organization of the 1948 Summer Olympics in London. In 1949 he left the Colonial Service and became the British Council representative in Australia until 1951. Back in England, he was chairman of the Royal Overseas League from 1955 to 1962.