Metropolitan Gerasimos was born in Kalamata, Greece to Nicholas and Anastasia Michaleas. He was the youngest of three children, with older siblings Peter and Kalliope. Upon successful completion of his elementary and secondary school education in Greece, he went to Canada to live with his brother for a few years, then immigrated to the United States. In 1970, he enrolled at Hellenic College in Brookline, Massachusetts, where he earned a Bachelor of Arts in 1973, followed by a Master of Divinity in 1976, both degrees conferred with high honors. Upon graduation, he worked with the school's new president, Dr. Thomas Lelon, and assisted in upgrading the infrastructure of the administrative services of Hellenic College Holy Cross to include the Office of the Registrar, Office of Student Affairs, and Office of Counseling and Guidance.

He was ordained to the diaconate in 1979 and was called to serve as Archdeacon to His Eminence Archbishop Iakovos, a position he held until 1996. Concurrently, he was appointed Dean of Students at Hellenic College/Holy Cross from 1980 until 1984. As Dean, he created the "Orthodox Actionline," the first Archdiocesan telephone ministry directed towards those who do not have substantive ties to the Church. Eventually, this ministry became an official part of the Office of Field Education at Holy Cross. He returned to the Office of Student Affairs in the early 1990's and remained connected to the school until 1996.

Also during his tenure as Dean of Students at Hellenic College/Holy Cross, he became centrally involved in the success of various programs benefiting future priests of the Greek Orthodox Archdiocese of America and their families. Specifically, he organized and monitored programs that evaluate and assist individuals at Hellenic College/Holy Cross with development in both their emotional and spiritual growth. These programs have been modified to suit the needs of the institution's current student population.

In 1984 he enrolled in the Master's program in Counseling and School Psychology at Boston College. After receiving his degree in 1986, he entered the doctoral program. He received his Doctorate in Counseling and School Psychology in 1993. His doctoral dissertation is titled, "Intellectual Deficiencies in a Substance Abuse Population."

While maintaining his position as Archdeacon and Dean of Students, he was appointed a member of the staff of the Outpatient Clinic of the V.A. Medical Center in Boston, a post he held until 1996. In 1998, he became Director of Admissions and Records at Hellenic College/Holy Cross. In 2000, he became Administrative Assistant to the Rev. Nicholas Triantafilou, President of Hellenic College/Holy Cross, until he was elected Bishop of Krateia by the Holy and Sacred Synod of the Ecumenical Patriarchate in December of 2001. He was appointed by His Eminence Archbishop Demetrios as Chief Secretary of the Holy Eparchial Synod of the Greek Orthodox Archdiocese of America. As Chief Secretary, he assisted the Synod in the promulgation and realization of its decisions.

In the area of academic achievements, he taught courses in Psychology at Hellenic College, as well as courses on Teleturgics at Holy Cross. Since 1985, he has been Senior Lecturer in the field of Personality and Psychology at Northeastern University, University College. The University Press of America published his dissertation in 1994. He is the author of a number of articles published in periodicals in the area of psychology, and is a member of the APA (American Psychological Association) and the ACA (American Counseling Association).

On February 22, 2005, the Holy and Sacred Synod of the Ecumenical Patriarchate elected him to be the new Metropolitan of San Francisco, with his enthronement being held on April 2, 2005. Since that time, Metropolitan Gerasimos has served the faithful in the Metropolis of San Francisco, focusing much time and effort on the expansion of ministries and the development of new programs, including a comprehensive Strategic Plan for the Metropolis of San Francisco so that it can be proactive in developing meaningful programs and ministries for the spiritual, educational, and social edification of its faithful. The plan has provided a roadmap for the ministries of the Metropolis, and many new programs are being implemented as part of this effort.

Another important aspect of the ministry of Metropolitan Gerasimos is a collaborative partnership with the Canonical Orthodox Bishops of the West. Regular meetings, annual Pan-Orthodox Clergy retreats, and the joint celebration of the Sunday of Orthodoxy are cornerstones of this effort. In addition, Metropolitan Gerasimos maintains strong Ecumenical relations especially with the Roman Catholic churches in the west, and has participated in various events including a pilgrimage in 2010 of Orthodox and Roman Catholic faithful to Rome, Athens and Constantinople.