I can see what you mean - example post 'check out these cars'
but what you see on top of the title is the 'featured image' of that post; and because the post has no content apart from the gallery shortcode, there is nothing shown below the title.
the content-gallery.php does not seem to get triggered to show the gallery images.
Twenty Twelve actually has not enabled the 'post-format' 'gallery';
for this, edit functions.php of your child theme;
add:
function twentytwelvechild_setup_post_formats() {
// This theme supports a variety of post formats.
add_theme_support( 'post-formats', array( 'gallery', 'aside', 'image', 'link', 'quote', 'status' ) );
}
add_action( 'after_setup_theme', 'twentytwelvechild_setup_post_formats', 20 );
then for each post with a gallery shortcode, edit the post, and tick 'gallery' under 'Format' in the right column (below the 'publish' box).
this sets the post-format of that post to 'gallery' which then gets used in index.php and single.php to call the content-gallery.php template part.
instead of a copy from Twenty Eleven, try this code for content-gallery.php in a child of Twenty Twelve: http://pastebin.com/7WbDs9v4
possibly add this to style.css of your child theme (it is copied from Twenty Eleven):
/* =Gallery
----------------------------------------------- */
.format-gallery .gallery-thumb {
float: left;
display: block;
margin: .375em 1.625em 0 0;
max-width: 100%;
}