Forecasters say the storm could hit anywhere from the Florida Panhandle to Texas as a major hurricane next week, though caution it is difficult to predict a storm's track and strength several days in advance.
"Let me say to the people of Mississippi: This is not a time to panic, but it is a time to get prepared," Barbour said at a news conference.
Roughly 2,700 government-provided trailers remain occupied along Mississippi's 70-mile coast, which was badly hit by Katrina three years ago. Officials are concerned because trailers are vulnerable to damage in high winds.
Barbour says mandatory evacuation notices are going out to trailer residents on Saturday. Evacuations of trailers and mobile homes in Mississippi's coastal Harrison and Hancock counties will start Sunday morning and will begin Monday in Jackson County.
The order also applies to some of the 2,800 cottages built as alternatives to trailers. Only those cottages in flood zones are subject to mandatory evacuation.
Barbour said it's too early for Mississippi to decide if a wider evacuation order will be needed. Those decisions will be made in the coming days.
"The blessing, if there is one, is that it's a three-day weekend," he said. "The storm is a long way off. People have a lot of time."
Mississippi was the first state to call for any mandatory evacuations in advance of a possible hit by Gustav. The mayor of Grand Isle, La., a community typically among the first to vacate when severe weather threatens, called for a voluntary evacuation beginning Friday afternoon. New Orleans was organizing buses in case an evacuation was necessary, but had not yet called for residents to leave.
U.S. Homeland Security Secretary Michael Chertoff said federal officials defer to state and local authorities on making evacuation calls, but can help them synchronize their plans.
"We try not to pull the trigger too early on evacuation because you don't want to have false alarms," he said.