Although filming on the upcoming DC superhero movie has finished, the 53-year-old actor - who voices the villain Doctor Phosphorus in the studio boss' Max TV show 'Creature Commandos' - has been added to the cast of the blockbuster in an undisclosed part.
In July, Gunn, 58, confirmed shooting on the film - which stars David Corenswet, Nicholas Hoult, Rachel Brosnahan, Isabela Merced and Milly Alcock - had been completed and the project had entered post-production.
Reflecting on his time working on the picture, the director said he felt "forever grateful" for the "goodness and kindness and love" he experienced while making the flick.
He wrote on Instagram: "And that's a wrap. God bless our cast and crew whose commitment, creativity, and hard work have brought this project to life.
"I set out to make a movie about a good man in a world that isn't always so much. And the goodness and kindness and love I've encountered on a daily basis on the set has inspired me and thrust me forward when I felt too spent to move on my own. Thank you all from the bottom of my heart. It has been an honour."
The filmmaker added: "The destination has been 'Superman', but the journey has been the toil and the laughter and the emotions and ideas and magic we've shared together on set - and for that I am forever grateful."
Meanwhile, Hoult revealed he had been "working out" to play the Man of Steel's arch-enemy Lex Luthor in the picture, which is slated to hit cinemas in July 2025.
During an appearance on the 'Inside of You' podcast, the 34-year-old actor explained the idea stems from a Superman comic, saying: "I have been working out. There's that bit in 'All-Star Superman' [comic book] where he [Lex] talks about his muscles being real and hard work and all that. I kind of took that as a little bit of fuel for the fire."