Girl Gun Lady (2021)

Both focus on high school students trapped in their school and forced to fight.

However, Girl Gun Lady has the characters going into an alternate reality in-show, while Kamen Rider Zi-O VS Decade is just full-blown set in a reality that is not the one from the main TV show.

Both shows entrust the lead characters with a figure/gun of a blue-haired anime girl that transports them into an alternate reality where they must then fight. But as things progress, things are actually more real than they seem.

Both titles are centered on a school setting with an all girl's school. Which the level of suspense it different, GGL focusing more on 'battles' whereas Zanokokuna focuses on 'audience approval', the actions of an individual affect the group and the reality of the situation affects the real world.

Both series focus on a female centered character group in a survival situation. There's a lot of suspense elements and character dynamics that feel very similar.

Both series have a female protagonist whose rather unassuming to start with. However, these same unassuming protagonists grow and change once they begin building, customizing and altering these ordinary puromo (plastic models) into their own unique pieces.

Both series are Female Centered Plot which focuses on competition in winning particular game or activities. Suugaku Joshi Gakuen is about girls who competes in Math and Girl Gun Lady is all about winning in the fight which is a survival game. If a competitor lose it would subtract their life span.

Although the premises are different there is a similar concept of life and death regarding how Girl gun lady and Gantz games are played.

The game impacts whether if the player gets to live or die in both parallel and the real world.

Both use guns as fighting weapons against a common enemy

Gantz has a darker and violent tone compared to Gun girl lady's toned down and less graphic violence.