Thanks for the reply.
In the plugin “Responsive Lightbox” there is a setting “Single Image as a gallery.” (By that I mean: Showing one image in the blog post or on the Page. If you click on the individual image, you see not one image in the lightbox. But a whole Gallery.)
I enabled this option “Single Image as a gallery.” in the plugin “Responsive Lightbox”.
But in the blog-post or on the page it shows me always the complete Gallery. So a gallery with many pictures (WordPress default Gallery).
How do I make, that I have only one picture, but that I display the contents of the Gallery in the lightbox?