Gray allowed five runs, six hits and three walks over 3⅓ innings at the Chicago Cubs on Thursday.
He is 5-3 with a 3.94 ERA in nine starts but has allowed 11 runs in his last two outings after giving up just nine earned runs in 41 2/3 innings over his first seven.
Gray was an All-Star last year, going 11-9 with a 2.71 ERA in 31 starts in his first season with the Reds.
Cincinnati selected the contract of right-hander Sal Romano, who had been on the taxi squad. Romano had not appeared with the Reds this season but was 14-19 with a 5.17 ERA in 67 games over the previous three seasons.