His departure comes only weeks after Geronimo founder Karen Harris left to set up a mentoring charity at the beginning of December.
Gilheany is now a director at Forster, heading up a team of 15 working on environmental sustainability, and corporate and international development clients.
He reports to CEO Jilly Forster and will work with clients including the WRVS, YWCA and the Fairtrade Foundation.
Gilheany said: 'I am keen to expand my experience in international development and environmental sustainability. These are new interests for me professionally, but not personally.'
He worked at Geronimo Communications for nearly four years as a director, working with clients including mentoring charity v, the Holocaust Memorial Day Trust and ethical search engine Everyclick.
His responsibilities will be taken on by director Anastacia Scott, who specialises in voluntary sector clients, as well as other members of the management team.
Geronimo MDs Laura Oliphant and Sarah Harris are now heading up the agency. Oliphant said there would be internal promotions in the next few weeks.