Gilbert is a city in Maricopa County, Arizona. Located southeast of Phoenix, Gilbert is home to 288,128 residents. It is the fifth-largest municipality in Arizona. Nearly a third of Gilbert residents are under the age of 18. As Gilbert approaches buildout, the estimate…Gilbert is a city in Maricopa County, Arizona. Located southeast of Phoenix, Gilbert is home to 288,128 residents. It is the fifth-largest municipality in Arizona. Nearly a third of Gilbert residents are under the age of 18. As Gilbert approaches buildout, the estimated population is expected to reach 330,000 people.