How to make Gallery Post?

Hello, Thank you for theme!
I wonder how can I make post with Slideshow Gallery in post thumbnail on main page?

I have chosen the “Gallery” post format in the Post Attributes box and uploaded the images via “Add Media”.
However, the post is published with text only.
I have tried centering images, choosing different sizes and adding them as WP Gallery – nothing works.
Is there any trick I’ve missed?