Any chance you are using GP 3.0 beta?
https://generatepress.com/generatepress-3-0-a-new-era/
If so we can use this filter:
add_filter( 'generate_get_the_title_parameters', function( $params ) {
if ( ! is_singular() ) {
$params = array(
'before' => sprintf(
'<h2 class="entry-title"%2$s><a href="%1$s" rel="bookmark" target="_blank">',
esc_url( get_permalink() ),
'microdata' === generate_get_schema_type() ? ' itemprop="headline"' : ''
),
'after' => '</a></h2>',
);
}
return $params;
} );
Adding PHP: https://docs.generatepress.com/article/adding-php/
If not then you would need this solution:
https://generatepress.com/forums/topic/force-open-all-wordpress-blog-posts-in-new-tab-target_blank/#post-612849
Using 3.0 would be the better solution ๐