It was confirmed at the turn of the year that Gunn, the new co-chairman and co-CEO of DC Studios alongside regular collaborator Peter Safran, would be ditching Henry Cavill's iteration of the Man of Steel for someone younger in his reboot.
This also meant that Amy Adams - the Lois Lane to Cavill's Superman - was out of a job.
In an update posted to Twitter on Saturday, The Hollywood Reporter's Borys Kit confirmed that moves were being made.
He wrote: "Meanwhile, right now on the Warners lot, James Gunn is screen-testing Nicholas Hoult, David Corenswet and Tom Brittney + Emma Mackey, Phoebe Dynevor and Rachel Brosnahan, in full wardrobe & makeup, for the roles of Clark Kent and Lois Lane for SUPERMAN LEGACY."
Hoult has previous with final-stage superhero auditions after losing the Batman role to Robert Pattinson, while Grantchester actor Brittney and We Own This City's Corenswet face landing their biggest breaks yet.
Mackey has continued to shine in Sex Education as Maeve Wiley, which lead to this summer's gig in Barbie, yet Bridgerton favourite Dynevor could bring fresh dimensions to the character of Lois.
Brosnahan, on the other hand, will be right at home in the company of Gunn after playing wannabe comedian Midge across five seasons of The Marvelous Mrs. Maisel.
Back in January, Gunn was forced to deny rumours that his new Superman had already been found.
"No one has been cast as Superman yet," he tweeted. "Casting, as is almost always the case with me, will happen after the script is finished or close to finished, and it isn't.
"We'll announce a few things in not too long, but the casting of Superman won't be one of them."
Watch: James Gunn appears to criticise Gene Hackman's Lex Luthor performance