The gallery post format use native WordPress functionality and it isn’t partially clear to see how to add images at first…but once you know then its easy.
To add images to your gallery post, click on the Add Media button. Upload an image. Normally then you would click Insert into post button but you not need to do this, just simply close the media pop-up OR continue to upload more images.
By uploading a image from within the post, WordPress automatically attaches this image to the post and then displays the attached images within the post in the gallery slider.
Please note though that an image can only be attached to one post for this to work.
If you go to the Media section of WordPress and look at the media list, in the Uploaded to column, it tells you which post the image is attached to.
After uploading some images to the post, go and check out your post and you will see all of your images being displayed in the gallery slider at the top of the post.