functions.php

<?php

/**
 * List post headings
 * @see https://www.billerickson.net/access-gutenberg-block-data/
 */
function be_list_post_headings() {
	global $post;
	$blocks = parse_blocks( $post->post_content );
	$headings = array();
	foreach( $blocks as $block ) {
		if( 'core/heading' === $block['blockName'] ) {
			if( false !== strpos( $block['innerHTML'], 'id=' ) ) {
				$element = 'h' . $block['attrs']['level'];
				$title = str_replace( array( '<' . $element, 'id="', '</' . $element ), array( '<a', 'href="#', '</a' ), $block['innerHTML'] );
			} else {
				$title = wp_strip_all_tags( $block['innerHTML'] );
			}
			$headings[] = $title;
		}
	}
	if( !empty( $headings ) ) {
		echo '<ol class="table-of-contents">';
		foreach( $headings as $heading )
			echo '<li>' . $heading . '</li>';
		echo '</ol>';
	}
}
add_action( 'genesis_before_entry', 'be_list_post_headings' );

Unfortunately there's a bug in Gutenberg now that only fills $block['attrs']['level'] for H1 headings.

Hopefully the bug will be resolved soon. There's no good reason for WordPress to not include the heading level in there.

Same issue with https://github.com/sortabrilliant/guidepost

No column support due to gutenberg bug.