Gray, Robert Stephenson, Michael Lorenzen and Raisel Iglesias combined for a two-hitter. Iglesias surrendered Kolten Wong's check-swing RBI double in the ninth before retiring pinch-hitter Matt Wieters on a fly ball to center to finish his 24th save.
Nick Senzel and Eugenio Suárez each drove in a run for Cincinnati, which stopped a four-game slide.
St. Louis had won five in a row to move into a virtual tie with Chicago for the NL Central lead. The Cubs had a chance to move in front again, but they lost 7-5 at Philadelphia on Bryce Harper's game-ending grand slam.
Dexter Fowler had St. Louis' first hit, a two-out single in the fifth. Gray then struck out Matt Carpenter to extend his scoreless streak to 18 innings.
Gray (8-6) walked three and hit a batter while throwing 97 pitches. Stephenson pitched the sixth inning and Lorenzen got five outs before Iglesias closed it out.
St. Louis had won Michael Wacha's last 13 starts against Cincinnati, dating to September 2014. He had earned the win in 10 of them, including each of the last seven.
Wacha (6-6) extended the St. Louis pitching staff's streak of consecutive scoreless innings to 22 before the Reds pushed across two runs in the fifth.
José Iglesias led off with a single and went to second when Tucker Barnhart was hit in the shoulder and head with a pitch. The runners moved up on Gray's sacrifice bunt, and Iglesias scored on Senzel's fielder's-choice chopper fielded by second baseman Kolten Wong behind the mound with no time to throw out Iglesias at the plate.
One out later, Suárez drove in Senzel with a line drive single to center.
TRAINER'S ROOM
Cardinals: Inflammation in OF Jose Martinez's right shoulder was clearing up enough for him to receive a cortisone shot later Thursday or Friday, manager Mike Shildt said.
Reds: 1B Joey Votto was scratched with low back tightness.
UP NEXT
Cardinals: RHP Adam Wainwright (8-8) allowed nine hits and seven runs in 3 1/3 innings in his last start against Cincinnati on July 19, a 12-11 St. Louis win.
Reds: Luis Castillo (11-4) has allowed seven hits and two runs over 12 innings in two starts against St. Louis this season. Cincinnati won both games.
