Inspired by https://gist.github.com/mzdakr/8416983

app.js

var urls = [
  'http://www.example.com/',
  'http://www.yahoo.com/',
];
 
var to = 'foo@example.com';
var sub = 'HTTP check failure';
var body = "The server seems to be down. Please check the server status.\n" ;
 
function myFunction()
{
  
  for (var i in urls) {
    if(check_server(urls[i]) == 200){
      continue;
    }
    MailApp.sendEmail(to, urls[i] + ':' + sub, body + urls[i]);
  }

  
}
 
function check_server(url)
{
  try{
    var res = UrlFetchApp.fetch(url);
    return res.getResponseCode();
  } catch(e){
    return 999;
  }
}