Firstly, Slimes and Winter Slimes are the only enemies in the game. I believe a new Ox enemy shall appear with the next update though I don't know when that will be.
Secondly I've unlocked everything in the gallery, so here's what I did. Scene 1 you need to move around in the wagon, not remain calm. Scene 2 is a Patreon exclusive; so if you are a Patreon, at night time, head to the Inn, click interact and a new line asking to investigate the back room will appear. And for Scene 3, while in a fight with a slime, click the submit button. There are two more scenes that are nice but don't appear in the gallery. When waking up in the cabin after the Wagon Scene, dont open the drawer and you'll see Mr. Alais sharpening his tool haha, and for the final one I discovered is actually pretty simple, click "take a peek" while in the hallway.
There you are my friend. Happy hunting.