Is it the gallery located at the bottom right of the main menu? If so, how do I unlock scenes? It doesn't seem to unlock a scene after I fought the slime
The gallery in this game is Celica's memories of scenes that are only available once in the story. But in the gallery, you can play them any number of times. This will be available in the Dream World, which opens later in the story (you will need to try to leave that world without your interlocutor's help). Scenes with monsters are always available in their locations, so they are not added to the gallery.