Gordon Forlong was the second son of William Forlong, a merchant, and his wife, Crawford Gordon. He was born on 14 February 1819, according to family sources in Pollok Castle, Renfrewshire, Scotland, and raised in the home of his great-uncle, Sir Robert Pollok, a member of the Scottish Episcopal church. Gordon was educated at Glasgow Grammar School and the Universities of Glasgow and Edinburgh. Influenced by deist ideas, he resisted pressure to enter the ministry of the church. He was admitted to the Scottish Bar about 1842, but found himself unsuited to narrow legal work.
In the late 1840s Forlong founded a 'Bank of Character and Skill' – a labour exchange for skilled workers – and about 1851 was invited to develop a similar body in London. While seeking sponsors he met an evangelical merchant who sought to convert him. Despite Forlong's deist views, he believed that like all Scots he was a Christian, and the merchant's challenge to him to talk as eloquently about Christ as he had done about his charity work led to his evangelical conversion.
At Margate, Kent, on 15 January 1852, Forlong married Laura Isabella Anstey. They had two daughters, Clara and Amy, before Laura's death in 1854. Forlong became active as a lay preacher and evangelist, and was caught up in the religious revival of 1859–60. Returning to Scotland with his second wife, Elizabeth Anna Houlton, whom he had married at Paddington, London, on 9 June 1857, he threw himself full time into revival preaching. Forlong was a gentleman evangelist, a lay preacher using his learning, but with an essentially simple message of ruin, redemption and regeneration. Influenced by Brethren concepts, which were endemic in the Irish and Scottish revival, he taught the pre-millennial return of Christ, and rejected ordination. He gained some prominence, and his message was popular with people in many parts of Scotland. His preaching also took him to England, eventually back to London, where in 1868 or 1869 he was involved in the establishment of Talbot Tabernacle in Notting Hill. A large working-class congregation enjoyed his preaching until he suffered a breakdown in health in 1876.
Forlong and his large second family (eventually numbering 11 children) departed for New Zealand in May 1876 on the Dunbritton, arriving in Wellington on 4 August. Forlong soon purchased a farm just north of Bulls. By March 1877 he was holding frequent Sunday services in the Bulls town hall, and speaking at various churches. He was reckoned the 'great sensation of the district', and converts, including a significant group from Rongotea, travelled some distance to hear him. Forlong soon had invitations from further afield, and in 1880 he moved to Dunedin to preach at the new Great King Street Congregational Church, and at a mission in the Queen's Theatre. He appealed to conservative Dunedin residents unhappy at the liberal élite's tolerance of freethought and evolutionary theory. Forlong vigorously challenged the freethinkers. His grand style, his classical allusions and his simple biblicism were mocked by some, but there were many conversions. Forlong left Dunedin in July 1883. He resumed preaching in Rangitikei (in co-operation with the Salvation Army) and gained many converts, moved on to Wanganui about 1887, and remained in the vicinity until about 1905. His wife died at Wanganui in November 1894, and in his old age he lived at Rongotea, where he died on 30 August 1908.
In a country where denominationalism was strong, Forlong was essentially an independent evangelist. He supported the Salvation Army, and his converts, organised by his former steward, James Chrystall, began the Open Brethren movement which spread outwards from Rongotea and Bulls. Forlong was cautious about the exclusiveness of this movement, and was unwilling to throw in his lot completely with it. He encouraged his converts to support all evangelical enterprise.
Forlong was a fundamentalist and a warrior in religion: his motto was no compromise, no quarter, and no surrender. At his funeral he was described as the last of the Puritans. A childhood accident had left him lame, but he was a striking and powerful preacher. His detailed but somewhat narrow learning (evident in his many published pamphlets), his passionate advocacy of the truth of the Bible, his zealous evangelism and his friendship with a wide range of fellow-minded people made him the most notable evangelist of his time, and a legend in evangelical circles.