Torsten Liebig

That did the trick. I added the filters to pre_get_posts like so:

add_action( 'pre_get_posts', 'add_custom_sql_filters' );
function add_custom_sql_filters( $query ) {
    if( ! is_admin() && $query->query["post_type"] == "gd_place" ) :
        add_filter( 'posts_join', 'geodir_filter_widget_listings_join', 10, 2 );
        add_filter( 'posts_where', 'geodir_filter_widget_listings_where', 10, 2 );
    endif;
    return $query;
}

which works like a charm. Thanks!