Rider Time: Kamen Rider Zi-O vs Decade (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.

Zhang Jing Yi is the FL in both. The chemistry is natural. The relationship had depth. However, BEITD does not have a satisfactory ending.

Both feature Kamen Riders from multiple series
Both have a murder mystery to be solved
Both are spinoffs from a main series