I’ve created this post as a user with the Author role, and in doing that I’ve also been able to add the gallery of two racing car images you can see beneath the main menu.
To do that, I used the hooked editor which was available to me on the Edit Post screen, below the main post content editor. As you can see, it’s just a normal WP Editor, but with the title Hooked Editor: Images after Main menu and some instructions. Using that editor, I could add a gallery of images in the normal way – just as if I was adding a gallery in the content of a normal post.
That explains how I added the gallery. To see how the hooked editor appeared there in the first place, check out the Image under the main menu example. I’m using exactly the same hooked editor in this example as I do in that one – and there I give a full explanation of how an administrator can create the editor for both.