If you get an HTTP error pop-up message on the screen while you are working on the Gmail app, then you should be aware that you do not select a wrong web page. Because this error message occurs if there is something wrong with the webserver. So, you should reconfirm what you select or try to reload the webpage and also check the internet connection. If you still troubled the same, then  contact Gmail by phone  for a quick fix. https://www.monktech.us/Gmail-Help-Phone-number.html