Now, filming has continued on the hit CBS drama, and cast member Aisha Tyler shared with PEOPLE how the set has changed in Gibson's absence.
"Thomas was an incredible part of the show for a very long time," Tyler, who plays Dr. Tara Lewis on the show, said of Gibson. "I know fans are going to miss him."
"It's a show that's been going on for a long time and a lot of the team has been together for a long time," she added.
"It's a show that fans love. They connect with the stories. They connect with the theme of the show, which is about smart people out there trying to make the world safer for everybody," the actress explained. "That hasn't changed."
While Tyler admitted that she does know how the show's writers are planning to handle Gibson's departure, she couldn't reveal anything beyond that.
"I know what's going to happen, but I cannot tell you," she said. "I do know what's going to happen, but I'm sworn to secrecy."
Criminal Minds will return for its 12th season Sept. 28 on CBS.