Access or edit custom generated permalinks with WP_Rewrite

Query vars don’t end up in $_GET, use get_query_var() to fetch or check the value of your custom query vars set by rewrite rules.

Your rule is currently returning employees as employees_slug because your rule has two capture groups, so employees_slug is held in $matches[2], not $matches[1]. The first capture group is unnecessary though, so the parens can be removed.

To apply a template, you can filter page_template and check for the presence of your query var.

function wpd_employees_page_template( $template ){
    if( get_query_var( 'employees_slug' ) ){
        $template = locate_template( 'employees.php' );
    }
    return $template;
}
add_filter( 'page_template', 'wpd_employees_page_template' );

Also note that a slightly simpler way to add a rule is via add_rewrite_rule hooked to the init action rather than filtering rewrite_rules_array.