New Orleans' mayor, LaToya Cantrell, declared her own state of emergency and closed all non-essential government offices.
The Mississippi Emergency Management Agency warned of storm surges of between 3 and 5 feet and told South Mississippi residents to be prepared to evacuate.
Gordon was generating winds of 65 miles per hour (105 km per hour) on Tuesday as it steamed west-northwest at 17 mph, the National Hurricane Center said.
As of Tuesday morning, it was located about 230 miles east-southeast of the mouth of the Mississippi River, the Miami-based weather forecaster said.
U.S. oil producer Anadarko Petroleum evacuated workers and shut production at two offshore oil platforms on Monday, and other companies with production and refining operations along the Gulf Coast said they were securing facilities.
The U.S. Coast Guard also said the ports of New Orleans as well as Gulfport and Pascagoula, Mississippi, may have to close within 48 hours.
Last year, powerful hurricanes hit Texas, Florida and Puerto Rico, causing thousands of deaths, hundreds of billions of dollars worth of damage and massive power outages.
The storm passed over Florida's southern tip on Monday afternoon. There were no reports of any injuries or deaths or any damage to buildings, said Alberto Moscoso, a spokesman for the Florida Division of Emergency Management.