Code.gs

function purge() {
  var promotions = GmailApp.search("category:promotions")
  for (var i = 0; i < promotions.length; i++) {
    Gmail.Users.Threads.remove('me', promotions[i].getId())
  }

  var forums = GmailApp.search("category:forums")
  for (var i = 0; i < forums.length; i++) {
    Gmail.Users.Threads.remove('me', forums[i].getId())
  }

  var updates = GmailApp.search("category:updates")
  for (var i = 0; i < updates.length; i++) {
    Gmail.Users.Threads.remove('me', updates[i].getId())
  }
}