As of the United States 2000 Census, there were 4,698 people (now over 4,800), 1,683 households, and 1,216 families residing in the city. The population density was 2,970.5 people per square mile (1,148.0/km²). There were 1,763 housing units at an average density of 1,114.7/sq mi (430.8/km²). The racial makeup of the city was 72.26% White, 0.72% Black or African American, 0.98% Native American, 1.51% Asian, 0.06% Pacific Islander, 18.97% from other races, and 5.49% from two or more races. 35.08% of the population were Hispanic or Latino of any race.
Gustine is an ethnic enclave of Portuguese-Americans. Most can trace their ancestry back to the Azores, but the Portuguese-speaking community also welcomes many families from mainland Portugal and Brazil. This is shown by a large turnout in the yearly OLM (Our Lady of Miracles) Portuguese Festa when over 20,000 people from around Gustine and far away come to visit.
There were 1,683 households out of which 36.2% had children under the age of 18 living with them, 56.3% were married couples living together, 11.8% had a female householder with no husband present, and 27.7% were non-families. 24.7% of all households were made up of individuals and 15.0% had someone living alone who was 65 years of age or older. The average household size was 2.79 and the average family size was 3.34.
In the city the population was spread out with 30.2% under the age of 18, 8.2% from 18 to 24, 25.6% from 25 to 44, 20.0% from 45 to 64, and 15.9% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 94.9 males. For every 100 females age 18 and over, there were 88.8 males.
The median income for a household in the city was $38,824, and the median income for a family was $45,583. Males had a median income of $35,920 versus $22,149 for females. The per capita income for the city was $16,821. About 11.5% of families and 16.9% of the population were below the poverty line, including 22.5% of those under age 18 and 15.4% of those age 65 or over.
Much of the town's income traditionally comes from dairy production and processing: Gustine is the home of a Golden Valley Cheese factory and formerly a Carnation processing plant. As agricultural land becomes covered by housing developments, Gustine is becoming a bedroom community to the San Francisco Bay Area, a 1-1/2 hour commute away.