You can watch them on Crunchyroll. Except for Gantz. I don't know where you can find that one. They had it on Xbox video for the xbox 360 a long time ago. It shouldn't be too hard to find, since its the most well known of all those. There were even live action movies of Gantz, as well as a video game on the Playstation 2. Devil Survivor 2 is based on a Nintendo DS game of the same name. Its part of the Shin Megami Tensei storyline.