He's not actually a "character" in the traditional sense, but rather a fictional serial killer known as the "Riddler". The *Criminal Minds* team is tasked with catching him across multiple episodes.
Here's what happened to Gideon:
* He was killed off the show after Season 6, with actor Mandy Patinkin choosing to leave the series.
* His departure was explained in the episode "The Fight", where he left the BAU after a personal crisis and emotional exhaustion from the nature of his work.
* His death wasn't explicitly shown on screen but it was implied in later seasons, as the team referred to him with phrases like "he's gone" and "he's not here anymore".
It's worth noting that Patinkin's departure from the show was controversial among fans, and some believe Gideon's death was poorly handled. However, it's clear that Gideon's story on *Criminal Minds* came to an end, though his presence is still felt throughout the series.