The recent SS with Idris and others has as their grand CGI battle the SS vs Staro and it was played for laughs.
In Guardians of the Galaxy every action scene is played for laughs.
So Gunn has no idea how to do serious action and that's what Superman requires.
Which Snyder understood on a fundamental level, but Gunn doesn't
So he's probably going to do Superman Returns part 2 with a smidgen of action and it's going to be weak.
And no matter what Gunn does with this movie nothing will compare with Superman's first flight from Man of Steel or his fight vs Faora and Zod