Gilbert had been suspended for four games for violating the league's policy on performance-enhancing drugs.
Harrison, 39, had 80 1/2 sacks in a Pittsburgh uniform, and he had at least five sacks in each of the previous three regular seasons even though his playing time was reduced.
He played in just five games this season and had one sack.
Harrison's agent, Bill Parise, said Harrison, who signed a two-year contract in March, wants to continue to play.
"There was no animosity or bad feelings," Parise told ESPN. "It's just the business of the NFL. I believe he still wants to play. We'll have to wait and see what happens with waivers."
Harrison was the 2008 NFL defensive player of the year and was named to the Pro Bowl five times in his 14-year career.
Harrison played just 40 snaps through 14 games this season after playing 587 snaps last year.
For his career Harrison has 82 1/5 sacks, including two with Cincinnati, as well as eight interceptions.