It's being reported that Gunn named Cena, Xolo Maridueña, and Viola Davis will remain in their roles. The three played Peacemaker, Blue Beetle, and Amanda Waller in the DCU (formerly DCEU), respectively. It's unclear if these three are the only ones carrying over to Gunn's regime.
The DCU is beginning to take shape. Gunn recently spoke about the canon of the series, making it clear that anything prior to the animated series Creature Commandos is non-canon. Superman: Legacy will kick off the DCU on the film side.
Prior to taking over the DCU with Peter Safran, James Gunn was known for his work in the MCU. He directed the Guardians of the Galaxy trilogy and the holiday special. He also directed The Suicide Squad and will direct Superman: Legacy. Additionally, he created and directed episodes of Peacemaker.
John Cena was introduced as Peacemaker in the Gunn-directed The Suicide Squad. He'd lead the self-titled spin-off series as well. It wasn't clear what his DCU status was, but evidentially, Cena and Gunn enjoy collaborating together.
Viola Davis made her DCU entrance in Suicide Squad (2016) as Amanda Waller. She'd appear in the sequel and in Black Adam and Peacemaker in an uncredited cameos. She will voice the character in Creature Commandos as well.
Xolo Maridueña made his DCU debut in Blue Beetle as the titular hero. He was previously known for his role on Cobra Kai. Given his young age, it makes sense that Gunn wants to keep the young star for the long haul.