The new media gallery functionality is supposed to do what the regular media storage did, but with an enhanced UX and design. The images uploaded as category images will be displayed in here, as well as any image uploaded via the content editor (WYSIWYG) as part of an attribute content, a cms block or cms page content or by the page builder.
For products, only images which are uploaded in the content of attributes via the content editor (say inside the product description) will show here, and not the images set on the product as values for the base, small, thumbnail images or additional images. This is how the old media storage worked and how the new one does too.
LE: if you don't see your images from the old storage, make sure you follow the instructions the official documentation, as you will need to start the
media.content.synchronization queue consumers for the initialization of the media gallery.