If you're looking for manga similar to Gad Guard, you might like these titles.
On the day of Eva, mankind was expelled from Ark City. Seven years later, mankind organized a combat unit consisting of a mechanized Rogel Geaks, which combines the power of super powers and cyborgs, to start a campaign to recapture the Ark City.
Yoi, a girl who has nowhere to go because she looks different from the people around her. Who reached out to her...?
At the end of the 20th century a fierce war raged between the Monsters and the humans. The outcome was devastating for humanity -- as the Monsters had won -- and thus a new era of Monster-ruling began. These Monsters feed on children and can turn any human who sees them into either a puppet or stone -- except for the Melos Warriors who are the only ones able to fight them. Meet Bokka, a young aspiring Melos Warrior whose only goal is quite simple: to save the Melody of Oblivion and vanquish the Monster King, once and for all!
The Queen anthology series focuses on the female characters from Code Geass, and presents side stories by an eclectic group of manga artists.