Gradkowski, 30, is a former fourth-round pick of the Ravens back in 2012. The Panthers signed Gradkowski to a three-year, $3.15 million contract in 2016, but he was placed on injured reserve back last year and later released with an injury settlement.
Gradkowski had a brief stint with the Jets during the preseason but was released at the start of the regular season.
In 2016, Gradkowski appeared in 10 games for the Panthers and was rated by Pro Football Focus as the No. 33 center out of 37 qualifying players.