I see you are using the gallery post format for the two posts I see. The content-gallery.php file is coded to only show the content when viewed in the full post view, it would then require you to make a child theme and then do some custom coding to change the content-gallery.php file. The default code is:
<?php if ( ! is_single() ) :
// If not a single post, highlight the gallery.
if ( get_post_gallery() ) :
echo '<div class="entry-gallery">';
echo get_post_gallery();
echo '</div>';
endif;
endif;
if ( is_single() || ! get_post_gallery() ) :
/* translators: %s: Name of current post */
the_content( sprintf(
__( 'Continue reading<span class="screen-reader-text"> "%s"</span>', 'twentyseventeen' ),
get_the_title()
) );
Basically what that code is saying, is to load just the gallery part only when you are on the blog home page (where the post intro’s (summaries) are, but don’t show any post content (text). The second code part is saying, if you are not in the blog home page but you are viewing the full post, then show the complete content, including the gallery.
So this batch of code would require modifications to not only show some content in the blog home post summary, but changing the “the_content” code snippet to be “the_excerpt” so that it automatically will grab some content from the first paragraph of text as a summary.
The other option you have is to change your gallery posts to be standard post format and then after your gallery, insert a Read More tag from your editor.
Or were you wanting just text content as a summary, then show the gallery in the full post only?