Gillis scored to put the Chanticleers up by seven with 2:14 to play. Charleston Southern's Will Saunders answered with a 3, but perfect 10-of-10 shooting at the line gave Coastal Carolina its largest lead and sealed the win.
The score was tied at 36 at the end of the first half, and tied twice more in the second, but Charleston Southern never led after the break.
Elijah Wilson scored 13 points for the Chanticleers (11-3, 2-0 Big South), including 8 of their final 10 free throws. Badou Diagne added 14 points and 10 rebounds.
Saah Nimley scored 26 points to lead the Buccaneers (6-6, 0-1). Nimley had 18 points at the break, half of Charleston Southern's combined 36 points.