The Gmail application shows mail that opens by the user and is unread by the user. If the user opens emails will mark as complete. Once the mail opens, the user will open mail that comes from any sender. Opening that mail causes unread mail to mark as read. This count may increase day by day if you do not open the emails.  It shows all your mail regardless of whether you read it or not. Google stores email on its servers and also in Gmail accounts. This is the reason the Gmail application shows unread emails. It shows even if you haven't opened the email application for days.  Until completely deleted, all emails are available in the spam folder. It will always remind you of unread emails in your Google account.