I'll admit I was skeptical of the decision to make Gantz:O a CGI animated film instead of a 2D animated or live-action film. Considering Japan doesn't have the best track record adapting manga into CGI animated features my lack of enthusiasm for Gantz:O was well founded. However, the more recent trailers of Gantz:O have me fully convinced that Gantz is a manga that is best adapted in CGI.
Why it's better than 2D animation
Not only does Gantz creator Hiroya Oku utilize CG images in the manga but when visualizing the characters in his mind he imagines real humans in a live-action film rather than anime characters. While 2D animation is able to portray realism it is impossible for it to feel as solid and tangible as live-action or CGI animation.
Gantz actually was adapted into a 26 episode anime in 2004 but it's hardly a representation of what a 2D animated Gantz film would look like. To its credit, the Gantz anime did utilize CGI animation but that was mostly to save money and the anime in general greatly deviated from Hiroya Oku's original vision.
Why it's better than live-action
So why not live-action? Well actually Gantz has already been adapted in live-action as two Japanese action films one simply called Gantz and its sequel Gantz: Perfect Answer. However, neither of these films are able to faithfully adapt the source material because of the limitations of the Japanese film industry.
Creating the alien creatures and advanced machinery of Gantz is difficult enough to do in live-action but pulling off the action scenes is impossible with practical stunts. Also while the characters in Gantz do have realistic human anatomy their faces resemble anime so actors could never portray the characters accurately.
It becomes more apparent how good Gantz:O is at adapting its source material if you compare it to other CGI anime adaptations like Appleseed Alpha. While Gantz:O perfectly translates Hiroya Oku's art into CGI animation Appleseed Alpha hardly resembles Masamune Shirow's art. Gantz:O truly is as Hiroya Oku describes an "authentic representation of the original work".