The teams traded 3-pointers at the end of the first half with Fleming making one and Liam O'Reilly scoring one for Gardner-Webb (10-11, 5-3) which went to intermission down 31-30.
Charleston Southern started the second half with a 7-0 run and later pushed the lead to 52-40 when Keeling hit a 3 then followed with a jumper with 13:44 left. The Bulldogs responded with a quick 7-0 spurt, but went more than four minutes without scoring as the Buccaneers went on a 12-0 run and Gardner-Webb never recovered.
Mannion, who recently reclassified and is ESPN's No.11 overall player in the class of 2019, chose Arizona over Marquette.
He'll fit in with Sean Miller's plans for a faster tempo on offense.
The cream of the crop of American high school basketball will congregate in Colorado Springs from Oct. 5-7, as USA Basketball unveils its newly expanded Junior National Team minicamp in collaboration with the NBA, NCAA and NBPA.