Gantz story is something more one of multiple life stories with added action. It tends to drag everything out far more than is really needed. The first 4 - 5 episodes are the introduction with the next 5 being explanations on the lives of the next crew while keeping tabs on the surviving last crew. It is almost a text book definition of a filler.
Art is decent for the most part. Scenery is done well and the occasional done 3D CG scene is something not often seen in anime. However the angles for some of these CG scenes can make the animated characters look incredibly flat and out of place, being 2D. The major thing that kills the art is the real facial expressions. they rarely change from wide eyed shock or surprise.
Sound is adequately placed and used quite well. Even background music pieces are well used and fit the scene accordingly without being so loud as to be the only thing that gets your attention. While many object stock sounds are used, one tends not to care because either the scene is explaining something or you're too bored to notice.
The one thing that makes this seem like a filler anime is the characters. It spends so much time explaining the back story of all the characters in the second groups and only a little of one character from the first group, that there's only few real action sequences. Everyone's too soft or their personalities take a complete 180, far too easily.
Enjoyment was quite low. They drag on the first few episodes far more than needed and the next how ever many episodes, nothing interesting really happens. Then when they are called upon again towards the end, everyone's constantly changing perspective and it takes them so damn long to do anything, you get bored of that too.
