It's hard to imagine Kirkman wrote this, albeit 20 years ago. It starts off decent enough, manga tropes 101 and all, but the thing is, there is no real risk or challenge. Zack is invincible using the Geldarian Tech Jacket, he kills a gazillion aliens who are gas-based life forms, so he does not have to face the reality of war, and even when he gets back to Earth, when it would have been a great opportunity to use the relativity effect to turn everything upside down (which seems to be the case at first), turns out he has been gone a mere 6 months.
He bullies a criminal syndicate into leaving his family alone over the span of maybe 4 pages, then finds his family in a jiffy, and his dad even looks younger now, more like his older brother than anything.
There is absolutely zero drama or engagement for the reader, and it's kind of a pity, because the art is very serviceable, and the setup is ideal for telling a good story.
The later version of Tech Jacket (around 2013, I think?) is not much better, as it tries to get to the other end grit-wise, with no real plan, while ripping off Ben 10. All in all, this seems just one of many missed opportunities for the amerimanga genre.