Gellaw was born in Addis Ababa, Ethiopia. After earning his undergraduate and graduate degrees, he began his career in journalism in 1993 as a freelance writer focusing on human rights and political issues. In 1995, he was one of the founding editors of the independent newspaper Addis Express, which was forced by the government to close in early 1996. Gellaw was a senior reporter and columnist for the Ethiopian Herald, the only English daily in the country, from 1996 to 1998, after which he was exiled to the United Kingdom.

Gellaw started his work in London as a radio producer and broadcaster for Health Africa. From 2000 to 2004, he was managing editor of New Vision, a UK-based refugee e-journal. Since 2005, he has been a regular contributor and columnist for major Ethiopian online media outlets. In 2006, Gellaw founded Addis Voice, a popular Ethiopian online current affairs journal published in both Amharic and English. Addis Voice serves as a multimedia platform that spreads uncensored news, commentaries, and analyses that are otherwise uncirculated in Ethiopia. Since its launch in 2006—a year that saw the closure of 13 newspapers and the trials of nearly 20 journalists for treason in Ethiopia—Addis Voice has published thousands of news stories, commentaries, and other multimedia content.

Gellaw is the recipient of a number of awards for his work in journalism. His op-eds, articles, and interviews appear in many publications, including The Wall Street Journal, USA Today, The Guardian, The Far East Review, the Stanford Report, and Global Integrity’s The Corruption Notebooks 2008, a book written by leading journalists around the word that focuses on corruption and abuses of power.