If you live in or near NYC, you're in luck.
The Japan Society is holding an event called "Japan Cuts", which features some very interesting movies from Japan.
Including both Gantz and Gantz: Perfect Answer.
Both movies will be screened on Saturday, July 9th, with part one at 12:30 and part two at 3:00. And both movies will be shown in their original Japanese with English subtitles. It is being co-presented with The New York Asian Film Festival.
To see both movies, the ticket costs $20.00 for non members. If you are a member of Japan Society, though, the double ticket will only cost you $14. According to the website, tickets aren't on sale yet, but you might want to keep checking back daily to secure your purchases!
I wish I could go see it myself, but I'll actually BE in Japan on that day. Talk about dumb luck. ^_^;
EDIT: Actually, the $14.00 ticket price is for members, students and seniors. So if you bring your student ID, you'll get the discounted ticket.