Run as a bookmarklet

Drag it to your bookmarks toolbar, refresh gmail, and click

revert gmail

you really should look over the source first

Run in the console

Refresh gmail and paste the code below into the console

(function(){
  function search(root, matchers, maxDepth, depth){
    depth = depth||0;
    maxDepth = maxDepth||matchers.length;
  
    if(depth == maxDepth) return [root];
  
    for(var key in root){ 
      try{
        if(root[key] !== root && (!matchers[depth] || matchers[depth](key, root[key]))){ 
          var path = search(root[key], matchers, maxDepth, depth+1); 
          if(path){ 
            return [key].concat(path);
          }
        }
      }catch(e){}
    } 
  }

  function searchDom(root, match){
    if(match(root)) return root;
    var child = root.firstChild;
    while(child){
      var found = searchDom(child, match);
      if(found) return found;
      child = child.nextSibling;
    }
  }
  var settingsEl = searchDom(frames[3].document.body, function(el){ 
    return (el instanceof frames[3].HTMLDivElement) && el.getAttribute("title") == "Settings"
  });

  if(settingsEl){
    console.log("Settings element found:", settingsEl);
    if(settingsEl.getAttribute("aria-owns")){
      alert("Run this script before clicking the settings menu. Refresh the page and try again"); 
      return;
    }
  }else{
    alert("Reversion failed. Couldn't find settings menu element.");
    return;
  }

  var settingsParent = settingsEl.parentElement;

  var root = frames[0];
  var path = search(root, [
      null, 
      function(k,v){ return v && v.src == settingsParent}, 
      function(k,v){ return typeof(v) === "object"}, 
      function(k,v){ return v == settingsParent}]);

  if(!path){
    alert("Reversion failed. Couldn't find settings object");
    return;
  }

  console.log("Path found:", path);
  var settings = root;
  for(var i=0; i<3; i++){
    settings = settings[path[i]];
  }

  console.log("Settings found:", settings);
  console.log("Oa:", settings.Oa);
  if(settings.Oa === false){
    settings.Oa = true;
    alert('Settings menu updated. Click "revert to old look temporarily" in the settings menu to complete.');
  }else{
    for(var k in settings){
      if(settings[k] === false){
        settings[k] = true;
        break;
      }
    }
    alert('Settings menu probably updated. Click "revert to old look temporarily" in the settings menu to complete.');
  }
  return void(0);
  
})();