Get attachments but only from post gallery?

If you are referring to the shortcode, then you can use the get_post_gallery() or get_post_galleries() function to retrieve the gallery data such as ids (a list of IDs separated with comma) and src (an array of image URLs).

Sample code #1: (using get_post_gallery())

1234 is the ID of the post containing the shortcode.

$galry = get_post_gallery( 1234, false );
$images = [];
if ( $galry && isset( $galry["ids'] ) ) {
    $ids = wp_parse_id_list( $galry['ids'] );
    $images = get_posts( [
        'post__in'  => $ids,
        'post_type' => 'attachment',
    ] );
    echo 'Number of images in gallery #1:<br>';
    echo count( $images ) . '<br>';
}

Sample code #2: (using get_post_galleries())

1234 is the ID of the post containing the shortcodes.

$galrys = get_post_galleries( 1234, false );
foreach ( $galrys as $i => $galry ) {
    $ids = wp_parse_id_list( $galry['ids'] );
    $images = get_posts( [
        'post__in'  => $ids,
        'post_type' => 'attachment',
    ] );
    echo 'Number of images in gallery #' . ( $i + 1 ) . ':<br>';
    echo count( $images ) . '<br>';
}

So:

• Use get_post_galleries() to retrieve the data of all shortcodes in the post content.

• Use get_post_gallery() to retrieve the data of the first shortcode in the post content.