As gs.sleep() is undocumented, it is difficult to know quite how it works and if it ties up a thread unnecessarily (I suspect it does), so a good alternative if you're looking for an asynchronous wait server side is to use the schedule once script include (thanks to John Andersen for point this out).

gs.log('Started','Async example '+ Date.now());

var so = new ScheduleOnce();
so.script = "(function(){gs.log('I waited 5 seconds','Async example '+ Date.now())})()";
so.setAsSeconds(5);
so.schedule();

gs.log('Ended','Async example '+ Date.now());