@Simon- I finally got it.
Add the following code to your functions.php
add_filter( 'generate_navigation_search_output', function() {
printf(
'<form method="get" class="search-form navigation-search" action="%1$s">
<input type="search" class="search-field" value="%2$s" name="s" title="%3$s" />
<input type="hidden" name="post_type[]" value="post" />
<input type="hidden" name="post_type[]" value="topic" />
<input type="hidden" name="post_type[]" value="reply" />
</form>',
esc_url( home_url( '/' ) ),
esc_attr( get_search_query() ),
esc_attr_x( 'Search', 'label', 'generatepress' )
);
} );
If you want to make it faster, either comment out or completely remove the reply line:
// <input type="hidden" name="post_type[]" value="reply" />
It’s still not superfast having to search all of the topics, particularly if you have a large database.