Hi NickMo1
It's JA Content type plugin, JA Comics supports this Gallery content type. In backend >> content >>> articles >> you can create new article with type of 'Gallery', there will be 'Gallery' tab where you can add images and these images will show in slideshow like that page.
However, this content type is supported in specific template only. Implementing it in other template will require much of customization: both plugin and template 🙂