The Gamecocks (4-0) had three players in double figures as Downey scored 20 of his points in the second half.
Gardner-Webb (0-4) had lost its three previous games by a combined eight points, including an 80-76 setback at Oklahoma on Saturday.
Grayson Flittner led the Bulldogs with 19 points. Jonathan Moore scored 17 and Aaron Lin added 11 points.
Zam Fredrick chipped in 18 for South Carolina, including nine straight points, and Mike Holmes scored 10 of his 13 points in the first half when the Gamecocks built a 43-34 lead.
"Once one person gets it going, the defense tries to focus on that person and that allows other people to get it going," Fredrick said. "Our guys were wide open and they were able to take advantage of our opportunities."
The Bulldogs came in shooting 43.2 percent but were held to a season low 31.3 percent (21 of 67) against the Gamecocks.
"I really think the difference was our defensive effort, though not great all the way through we found a way to grind it out to get the game a little bit more where we wanted it," said South Carolina coach Darrin Horn. "The biggest thing we can take from this game is we had so many different people contribute in key ways."
The lead grew to as many as 16 points in the second half and the closest the Bulldogs got the rest of the way was eight points.