Kyoushoku Soukou Guyver (2005)

Zetman would probably not exists if Guyver didn't. Both anime feature similar plot, gore, monsters disguised as humans and tragic events. Both heroes have similar powers which is somewhat biological and also like the monsters powers. 

In both The Guyver and Zetman, the main character can transform into a strong humanoid being and must utilize their new superhuman abilities to protect mankind against monsters that can morph into humans. Both anime also deal with a secret organization that seeks to utilize their power for evil. They are also action-packed anime with plenty of violence and gore. If you liked one, you'll like the other.  

Monsters disguised as humans created by a secret organization and a a hero whit bio enhanced powers who fights whit them.  

-Monsters disguised as humans -Monsters are human made -A suit which gives you powers 

Both anime are series with the premise of monsters hiding inside human skin. Each consist of dark, grim stories about a protagonist (or in Zetman's case, protagonists) using superhuman abilities and technology to fight an evil organization of monsters. Each have major tragedy to the lives of all the main characters as the story unfolds, and the effects upon each character as it goes along. I personally enjoyed both series thoroughly and I think you will enjoy one if you like the other, although both Zetman and Guyver are small snippets of their respective stories, and left me wanting a bit more. 

These two anime focus on a main character that can transform into something that is not human and uses his superhuman powers to fight against evil. The main character in these two anime are fighting for their lives against powerful evil aliens. Not to mention these two anime are filled with violence, blood and gore too. 

Humans use alien technology to build suits that can either transform them into monsters or give them monster-like abilities.  

It's the whole Sci-fi aspect of it that brings the two so close together. They have similarities in some of the situations that the characters are placed in, creature designs, as well as a big company being responsible for most, if not, all of the world's problems. Though I did think that Generator Gawl ended better than Guyver, they both still had a wow factor about them. I recommend these to be watched in the English dubbing.  

These two anime focus on a male hero that can transform into something that is not human and must defeat an evil secret organization that plans on taking over the world. The Guyver is more violent compare to Generator Gawl, but they do have similar themes of not losing your humanity in the pursuit of justice.  

These two anime are remakes of classic ultra violent superheroes and adapted them into the modern world. Both of these two anime have a human boy transforming into a monster and must fight for their humanity. They both have crazy and violent action sequences while exploring the dark side of human nature. 

These two anime have a young boy that gain superhuman powers from aliens. The main hero in these two anime have to fight other evil aliens while fighting their humanity. The main hero in both anime have share a relationship with their powers too.