Gessi was born in Ravenna, in Romagna. He acquired his military experience serving in the volunteer corps of Garibaldi in 1859 and 1860. He was one of numerous Garibaldi volunteers who went on to be regular soldiers, not only in the new Italian army, but in several others.
He also fought with the British forces in the Crimean War (1854–55), where he first met General Charles George Gordon. Gordon later described him as "Italian subject, aged 49 (in 1881). Short, compact figure; cool, most determined man. Born genius for practical ingenuity in mechanics. Ought to have been born in 1560, not 1832. Same disposition as Francis Drake. Had been engaged in many petty political affairs. Was interpreter to Her Majesty's Forces in the Crimea, and attached to the headquarters of the Royal Artillery."
In 1876, while serving for General Gordon, he explored the course of the upper Nile (Bahr El Jebel) and mapped its descent from Lake Albert. He then ventured in the territory of the Oromo people, later becoming governor of the Egyptian Bahr-el-Ghazal province, where he struggled against Sebehr and the slave trade and also started the deployment of natural gum. Gordon regarded him, perhaps unfairly, as a brilliant mercenary - perhaps he did not come up to Gordon's standards of Noncomformist rectitude.
After his death, his son Felice published his father's memoirs in the book, Sette anni nel Sudan egiziano (Milano, 1891).
- Moorehead, p 168
- Zaccaria, Massimo: Il flagello degli schiavisti: (Romolo Gessi in Sudan, 1874-1881), Fernandel scientifica, Ravenna, 1999, p. 272; ISBN 88-87433-08-9, ISBN 978-88-87433-08-1