Where can I buy the digital version of Gantz manga? I searched in Dark Horse Comics site but they don't have the digital version. Amazon also just has the Physical version. Does Gantz not have a digital version of the manga?
Short answer, NO, there's no digital version of the US release of the GANTZ manga.
Long answer, some authors and publishing companies in Japan back in the day were against publishing their works digitally (like Nishio Isshin, Kawahara Reki, Log Horizon's author whose name keeps evading me, Kadokawa back in the day), I don't know for sure if that was the case with Hiroya Oku or Shueisha (I believe Shueisha was against it but don't quote me on that), but the fact is the Japanese digital version wasn't released until February 18th, 2014
This is 6 months AFTER the last volume was physically released in Japan, which came out on August 19th, 2013. All the volumes were released in bulk on bookwalker that very same day. The link here will take you to the full list of the GANTZ releases (site is in japanese). Nowadays, more and more older series are getting digital releases in Japan.
Which means that there was NO digital release of GANTZ in Japan back in 2007 when Dark Horse made the license deal. IF their original deal had contemplated the future possibility of digital releases in Japan, Dark Horse could legally sell the digital version, but there's been no such release so far so either they don't care or they can't release digitally without another deal.