The joke suggests that Marvel has found the perfect replacement for James Gunn named Jimmy Gunnbergo, who is simply just JJames Gunn with a mustache.
Jimmy Gunnbergo first appeared in a fake headline saying, “Marvel [Hires] First-Time Filmmaker Jimmy Gunnbergo to Direct Guardians 3.”
Hopefully whoever they find to replace Gunn is able to be as great of a choice as the likes of Jimmy Gunnbergo.
Following the firing of James Gunn from Guardians of the Galaxy Vol. 3, many people on the Internet have spoken out against his termination, including both fans and celebrities alike.