Conversations: Changing Subject, Bookmarks, and more
To help keep your view clean while viewing or composing a message, Gmail hides any previous replies that were copied into the message from earlier parts of the thread. So before replying to, or forwarding a message, it's a good idea to view this hidden text to see what's actually being sent to your recipient.
To see the entire contents of a reply you’re composing, click the small ellipsis icon to display the hidden text. (Click this icon again to re-hide the text).
This shows information from any previous replies that were copied into this message. If you don't want to send all this information in your reply, simply highlight and delete it.