This styling serves to provide some separation between your mail text and your signature.
Gmail Setting to Move Signature above Quoted Text in Replies and Forwards
Gmail users get a signature option on settings page to enable and add a rich signature at the bottom of every email.
By default, the signature appears at the bottom of the email even below the quoted text in replies.
The default signature placement is fine for new compose but I am sure many of us want to have the signature above the quoted text i.e. just after the recent message in case of replies and forwards.
Go to the Gmail Settings page and open General Tab.In the Signature option, you can see a new check-box available just below the signature box with text as “Insert this signature before quoted text in replies and remove the “–” line that precedes it”.
Put a check mark on it and click save changes at the bottom of the page.
Once you checked that box save your changes and now your signatures should appear above the marker "--" on replies and forwards.