How closely would you want this to integrate with other parts of the forum? But tbh, this sounds like a relatively simple extension. At the minimum, you'd need:
- Model setup for the media gallery items (models + endpoints + serializers)
- An admin page for creating new media gallery items
- A forum page for displaying media gallery items
From there, you could add more features / complexity:
- a permission for users to see the media gallery page
- a permission for users to add media gallery items (use the same API endpoints, but would need a frontend interface)
- whatever other functionality you desire