According to the United States Census Bureau, the city has a total area of 12.4 square miles (32.1 km²), all of it land.
There were 11,990 households out of which 40.4% had children under the age of 18 living with them, 62.4% were married couples living together, 9.2% had a female householder with no husband present, and 25.5% were non-families. 20.9% of all households were made up of individuals and 6.6% had someone living alone who was 65 years of age or older. The average household size was 2.70 and the average family size was 3.17.
In the city the population was spread out with 28.9% under the age of 18, 6.5% from 18 to 24, 31.7% from 25 to 44, 24.3% from 45 to 64, and 8.7% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 94.2 males. For every 100 females age 18 and over, there were 90.4 males.
The median income for a household in the city was $66,031, and the median income for a family was $85,917. Males had a median income of $51,391 versus $35,922 for females. The per capita income for the city was $29,040. About 2.2% of families and 3.7% of the population were below the poverty line, including 4.1% of those under age 18 and 4.4% of those age 65 or over.
In 1970 most of northern Gahanna was still unannexed portions of Mifflin and Jefferson Townships.

Gahanna also has an aggressive brownfields assessment and redevelopment program that includes the development of a community-wide brownfields inventory, the cleanup of a major abandoned landfill, and City-initiated assessments and demolition of abandoned or environmentally suspect properties.
The largest brownfields redevelopment project to date is the closure of the Bedford I Landfill, recently completed in late 2007. The closure is a result of a 2005 public-private development partnership with the non-profit Central Ohio Community Improvement Corporation to remediate and redevelop the abandoned Bedford I Landfill. The site will be transformed into Central Park of Gahanna, a major new business campus that will feature office buildings, golf academy, conference center, and retail businesses. Funding for this major project includes $3,000,000 from the Clean Ohio Redevelopment Fund, $200,000 from the US EPA, $150,000 from the Community Development Block Grant program, significant assistance with assessment, stormwater remediation, and staff time from Gahanna, and other investments by the Central Ohio CIC.
Gahanna has been listed as one of the Top 100 Best Places to Live in 2007 by Money magazine.