Jacksonville State (12-5, 4-2 Ohio Valley) shot 47.7 percent from the field and only took its biggest lead of the game after a Gaines free throw put the Gamecocks up 33-26 at the start of the second half.
Eastern Illinois led 48-45 with 7:08 remaining in the game but the Gamecocks went on a 8-2 run sparked by Brian Williams, who scored four straight buckets to give Jacksonville State a 53-50 lead and they never trailed again.
Williams ended his night with 16 points, seven assists and five rebounds.
Sherman Blanford had 10 points and seven rebounds to lead Eastern Illinois (3-15, 0-5). Taylor Jones and Austin Akers each scored nine points for the Panthers.