Gerretsen was born in Hilversum, Netherlands during World War II, and moved to Canada with his parents in 1954.

His son Mark Gerretsen served as a Kingston, Ontario city councillor and was elected mayor in 2010.

After retiring from municipal politics, Gerretsen served as the Chair of the Ontario Housing Corporation (1989–1992) and was a Deputy Judge in the province's Small Claims Court (1993–1995).

Gerretsen was generally regarded as being on the right-wing of the party, and achieved some support in his home base of eastern Ontario.

Gerretsen was the primary spokesperson for the McGuinty government in its controversial decision to permit further housing expansion on the Oak Ridges Moraine, despite an election promise not to do so.

His main legislative success has been in leading in the enactment of a 1.8 million acre (7,300 km2) green belt surrounding the City of Toronto in 2005.

He was appointed Attorney General of Ontario and was re-appointed when Kathleen Wynne became premier in early 2013.

[14] On March 19, 2020, amid the coronavirus pandemic, Gerretsen tested positive for COVID-19, with his wife Assunta testing positive earlier that week, after they were both repatriated to Canada from the Grand Princess cruise ship; they were quarantined at CFB Trenton.