Cincinnati Reds
The proximity argument remains for the Cincinnati Reds. They, along with the Cardinals and Braves, are one of the closest teams to Sonny Gray's hometown. In fact, Gray could have signed with the Reds following the 2023 season, but he opted to sign with the Cardinals instead.
Gray also has ample familiarity with the Reds' organization. He spent three years with the Reds from 2019-2021. During that time, he posted a 3.49 ERA with a 10.6 K/9 rate, the highest of his career for any team he played with. Gray also managed to limit the home run ball despite Great American Ballpark being a launchpad.
The Reds have one of baseball's best rotations. They are ranked tenth in staff ERA at 3.86, ninth in FIP (3.97), and eleventh in K/9 (8.60). They have a staff ERA- of 87, fifth best in the league. Their staff consists of Hunter Greene, a true ace at this point in his career, Nick Lodolo, Zack Littell, Andrew Abbott, and Brady Singer.
Sonny Gray would be the most experienced pitcher on this staff by far, thus allowing him to continue in his leader/teacher role for the Reds. There is ample talent among those starters, so Cincinnati would have a strong rotation as they look to compete more in the division and throughout the National League.
The Reds have experienced their fair share of injuries this year to starters, including Rhett Lowder, and Wade Miley. They'll also lose Zack Littell, who posted a 3.86 ERA in 182 innings with a 6.28 K/9, to free agency this winter. Trading for Sonny Gray to fill Littell's spot would be a decent upgrade for their rotation.
The Reds may be more financially strained than they would like if they trade for Sonny Gray. Their payroll this year is around $120 million, and I wouldn't suspect that their ownership group would be interested in adding significant payroll given their market size. Additionally, the Reds may want to spend money this winter on offense given their team wRC+ of 92, 26th in the league.
Cincinnati's closeness to Tennessee paired with Gray's familiarity with the organization makes it an ideal landing spot for the starting pitcher. If he's willing to accept a trade back to the Reds, the Cardinals could pluck from a Reds' farm system that features six top-100 prospects.