Hi,
You were almost there.
You did change the type of the menu item, but a category listing menu item needs an associated products listing module in order to display the products. Otherwise, it can only display categories.
If you went to the menu Display>Content menus and had edited the "gallery" menu item, you could see an error message telling you that no products listing module was associated to the menu item, with a link to do it automatically.
I've actually clicked on that for you and the products now display properly on your website.
You can now edit the menu item to customize the display of the products if necessary.