Yes it has been going some whre, haven't you been reading ?
Ok the replays of the scenario where they go and hunt down some aliens, this is where you might misinterpret that the story isn't going anywhere.
Your missing out the other parts of the story.
So, their stuck in that game and they don't know the rules. The next game they do a bit better. From the first time, they were told to score 100 points to be free, so that my dear friend is what the story is about.
Then later on they keep meeting new groups of people who mostly all horribly fail to survive because their a. unskilled, b. inexperienced c. too stupid c. too careless or unlucky
So then Katou and Kishimoto die, then Tae died.
Now Kei discovered that 3 things happen when you get 100 points, and 1 of them is ressurecting a person.
It may be a replay of the game, but the context is always different as their always fighting different alien targets, and lately things have turned even more bizzare with the appearance of nanobot vampires hunting the gantz participants down in turn. Not to mention Izumi who is apparently the only known person to have managed to beaten the game, and the other new interesting characters such as the country side general, the super star, and 2 psychics.
Where exactly is it not moving forward ? the whole game is what it is all about.
Look at Naruto, that revolves around ninjas, and one piece revolves around pirates, and etc.
Ok look at this from a Naruto analogy.
Remember that bit where it took 3-4 episodes for the Hokage to die ? Now that was straining it a wee bit too much.
But now lets look at Gantz. It is all about character development. It's about learning new things. Just look at their world and reality.
It's not like everything would be known to them so easily in the room with the black ball would it now.
It's a mysterious room where the rules are not all to apparent or a guide as to how to play it.
Most of the people that went to the room the first time were all pole axed by what the fuck was happening.
They didn't immediately perceive what was going on and so died. This was mostly due to very stupid people that couldn't adapt to the present situation at hand and all were to embeded in their previous lives to know wtf was going on now.
I think that what we know so far seems about right. Not too much knowledge and not too little and at the pace it's going i don't see what is wrong.
That just means more Gantz goodness for all don't you agree