filter-content-guterberg-blocks.php

add_filter( 'the_content', 'airpets_terms_nav' );
function airpets_terms_nav($content) {

    if( is_page_template( 'page-terms.php' ) ):

        $menu = '<div id="top" class="anchor-menu"><ul class="menu">';

        $blocks_arr = parse_blocks( get_the_content() );

        foreach( $blocks_arr as $block ):

            $text = strip_tags( $block['innerHTML'] );

            $slug = slugify( $text );

            foreach( $block as $key => $value ):

                if( $key == 'blockName' && $value == 'core/heading' ) $menu .= '<li><a class="btn" href="#' . $slug . '">' . $text . '</a></li>';

            endforeach;

        endforeach;

        $menu .= '</ul></div>';

        $content = $menu . $content;

    endif;

    return $content;
}