I dislike the idea of a movie, be it live action or animated. Here's why:
1) It will be too short for enough of Gantz's storyline to be fit in. The film would feel rushed; key parts would be left out.
2) Severe content edits would be required. Some of the material in Gantz...well, getting away with it in a film would be difficult. Considering Ichi the Killer, I suppose a lot can be done, depending on the country where the film is made...but I still get the feeling it would be greatly changed.
3) It would look silly in live action. I doubt if the aliens can be properly communicated on screen. Some of them just look...well, too ridiculous. It could be done, but when combined with the short length and content edits, I suspect the finished product would suck.
1) As with live action, it would be too short. Way too short.
2) What would they use for a story? There's already an anime covering through the Buddha arc, and just throwing the Dino mission into a movie seems like a waste to me.
I would rather see the rest of the story told through anime (or OVAs maybe?). As for movies, the only way I could see them working and being good is if they told the stories of the previous Tokyo hunters, or another country's.