The child, whose age and identity were not released, died when a tree fell on top of a mobile home.
With maximum sustained winds near 65mph (105km/h), Gordon is expected to make landfall later on Tuesday, the National Hurricane Center (NHC) warned.
Tropical storm Gordon made landfall just west of the Alabama-Mississippi border, the US National Hurricane Center (NHC) says.
More than 27,000 customers are without power Tuesday night as tropical storm Gordon began pushing ashore.
The center says Gordon will bring heavy rainfall to southwest Alabama, southern and central MS, southeastern and northeastern Louisiana and southeast Arkansas through late Thursdays. The storm was expected to slow down and move inland on the MS or Alabama coast by early Wednesday.
There are several disturbances tracking across the Atlantic over the coming weeks including Florence which attained hurricane status on Monday, making it the third hurricane of the Atlantic hurricane season.
UPDATE 10:15 a.m. Tropical Storm Gordon is honing in on the Gulf Coast and is expected to be only hours away from strengthening into a Category 1 hurricane.
In anticipation of heavy rains, the Flood Protection Authority-East (FPA) has closed 21 floodgates on the Lake Borgne Surge Barrier near New Orleans, according to a press release.
The storm was gaining strength and was expected to become a hurricane as it comes ashore late on Tuesday near the border between Louisiana and MS, and drop as much as 8 inches (20 cm) of rain in some areas of the US South still reeling from hurricanes a year ago. The National Hurricane Center calls the storm "life threatening". "Now is the time to get your plans all set", he said.
New Orleans Mayor LaToya Cantrell said the city has "the pumps and the power" needed to protect residents.
Louisiana Gov. John Bel Edwards, Mississippi Gov. Phil Bryant, and Alabama Gov. Kay Ivey all declared states of emergencies ahead of the storm's landfall.
The last hurricane to strike the US was Nate in October, coming ashore in Biloxi with 75 miles per hour winds.
No coastal watches or warnings are in effect through the weekend for Florence. "I think when you look at how rough the water is today, it's pretty self-explanatory".
"You just blend it into your lifestyle when you live outside the levee system".
Tropical Storm Warnings now include the western Panhandle of Florida.
Off to the east coast of Florida, storms are developing.