You need to install the plugin Webnus Gallery via this article and activate it. Install Plugins . Then a new menu will be added to WordPress dashboard panel called Gallery.
In Gallery submenu: Gallery shows all the posts.
In Gallery submenu: Add Gallery Item, you can add a new post.
In Gallery submenu: Gallery Categories, you can set categories.
Please note: To use Masonry galleries in your Galleries, you can use featured images with different height and fixed width. More height will stretch the image more.