I'm creating an event scene in NaniNovel,
and I want to know how to create and use a sub-script that will be used only in the event scene.
For example, there is a special conversation in the event scene,
and I want to display the same conversation later when viewing the event in the gallery.
Is there a way to do this?

The gallery is designed for CG textures only. You would need to create a custom gallery for any other type of content. Check the Custom UI example project in the samples repo which has an example of a music gallery that is made entirely without C# scripting: https://github.com/naninovel/samples