Once downloaded:

Before Dark Horse licensed Gantz, fan groups like Bakuman and Red Hawk Scans translated the series. These are often lower quality than the official release but easier to find via Nyaa (torrent) or MangaDex (online reader). MangaDex does not offer direct downloads, but you can use browser extensions to rip the images (though this violates their terms of service).

During its original run, groups like Miyako, OMT, and Red Hawk released fan translations. While historically significant, these are the lowest quality. Modern fan re-scans using the official Dark Horse translation are superior.