Taking to his Threads account, Gunn clarified when his DCU timeline begins. “Nothing is canon until Creature Commandos next year- a sort of aperitif to the DCU – & then a deeper dive into the universe with Superman: Legacy after that,” he revealed. “It's a very human drive to want to understand everything all the time, but I think it's okay to be confused on what's happening in the DCU since no one has seen anything from the DCU yet.”
He continued, “And, yes, some actors will be playing characters they've played in other stories & some plot points might be consistent with plot points from the dozens of films, shows, & animated projects that have come from DC in the past. But nothing is canon until CC [Creature Commandos] and [Superman] Legacy.”
James Gunn will have a tall task ahead. It sounds easy, but fans have been following one continuity for a long time (since 2013's Man of Steel, to be exact). He seems to understand that as his DCU slowly inches closer.
With holdovers incoming, that allows fan-favorite actors to return to roles. It's unclear exactly who and what storylines will continue, but expect more clarity soon. But it appears all films pre-Creature Commandos, including Blue Beetle (which introduced a new hero) will not be canon even if the star returns.
Prior to landing the DCU gig, Gunn was known for his work in the other major comic book movie franchise, the MCU. He directed the Guardians of the Galaxy trilogy (and Holiday Special) for the MCU.