I think the anger for Gunn comes from the fact that Gunn took over WB completely, renamed it, and instead of using the known actors that were portraying the characters, he went to rely on a script that was not his, and was a script that the prior regime had on a back burner...
Because Gunn could have used the current actors, and applied the vision that he had, and you could have seen Henry Cavill in this exact same suit, or a suit possibly similar to what we saw today... I do think that when Gunn was given the reins, he could have done something that would have mended the bridge that Walter Hamada and the past regime burned, but he didn't, and that's probably why these people hate Gunn so much... because there was potential, and instead of blossoming that potential, he gave up.
If he had mended that bridge, I guarantee you, we would not see the vitriol and hatred that Snyder fans have for Gunn, and I do believe that the fan base would have been more unified if Gunn took into consideration the fan base that the Snyderverse had, and still has to this day, And transitioned the DCEU into the DCU by finishing out what Snyder had originally planned... I really truly do believe if he did that