See https://ithoughthecamewithyou.com/post/email-alerts-for-new-referers-in-google-analytics-using-apps-script

NewReferers.gs

var TableId = 'ga:your-view-id';
var SendEmailTo = 'your-email-address';

function main() {
  var scriptProperties = PropertiesService.getScriptProperties();
  var currentProps = scriptProperties.getProperties();
  var anythingNew = false;
  var newText = '';
  
  var yesterday = Utilities.formatDate(new Date(new Date().getTime() - 24 * 60 * 60 * 1000), Session.getTimeZone(), 'yyyy-MM-dd');
  var options = {
    'dimensions': 'ga:fullReferrer',
    'filters': 'ga:medium==referral',
    'max-results': 20000
  };
  var report = Analytics.Data.Ga.get(TableId, yesterday, yesterday, 'ga:sessions', options);
  if (report.rows) {
    for (var i = 0; i < report.totalResults; i++) {
      if (!(report.rows[i][0] in currentProps)) {
        Logger.log('Found new referrer: ' + report.rows[i][0]);
        scriptProperties.setProperty(report.rows[i][0], report.rows[i][1]);
        anythingNew = true;
        newText += 'New referrer: ' + report.rows[i][0] + '\r\n';
      }
    }
  } else {
    Logger.log('GA report is empty');
  }
  
  if (anythingNew) {
    MailApp.sendEmail(SendEmailTo, 'Found new referrers for ' + TableId + ' on ' + new Date(), newText);
  }
}