Looks like the rating couldn't be higher because of (butthurt) Berserk fans who can't empathize with the very author that have bring them the very masterpiece they love. Here is the thing: go read Miura interviews and you know despite the grotesque content he is very sincere and pure hearted, and also very enthusiastic on his craft. He is the type of creator who never run out of ideas and very passionate with the story he wanted to tell.
Up to the latest chapter, I haven't seen any decreases in Miura's passion and dedication to his child, Berserk. However it is clear that before Gigantomakhia happened when the hiatus become severe he had stricken with a very bad author block. It is not because he couldn't continue the story, but rather due to severe creative burned out that coming out of unchanneled ideas. This is bound to happened because Miira's own conduct on doing heavy research and omnivorously gobbled all recommended material that was given by his friend to him. It is very obvious that there are a lot of research excess that he couldn't put into Berserk and have to be channeled somewhere. We are lucky that the we only got the overstayed buttmonkey pirates in the Sea God arc instead of something more severe. In that way, he is very similar to Hideo Kojima before the latter left Konami and finally create something that is not Metal Gear.
This is why despite its very short run, Gigantomakhia is jampacked with shit tons of stuffs, themes, and references. Miura tried his very best to channeled all of his creative ideas into this 1 volume story so he would no longer feel burned out after doing Berserk for the past 30 years. It is obviously not as good as Berserk, but for what is Gigantomakhia is very good as it had worldbuilding, theme execution, and pacing that could easily trump over longer and lesser manga.
If you are Berserk fans, try to read his interview. He is not a man who draw his own experience as the source of the awful stuff he had drawn and shown. He was in fact, feel into depression for weeks after Volume 13. It is honesty that makes Berserk very special to him and to us. If you wanted his magnum opus stayed that way, you better tried to be emphatic if he done another side project while Berserk still running.