A normal copy will not have that extra line. However, using NppExport to Copy HTML to Clipboard will.
That’s because when the NppExport plugin generates the HTML and puts it in the clipboard, it includes a <title>Exported from Notepad++</title> as part of the HTML it exports. (This is because Notepad++ is generating standards-compliant HTML, and a full HTML document requires a TITLE.)
There are no configuration settings for that simplistic plugin, so you cannot change the behavior. Sorry.
It’s simple enough to delete that line after you paste the results into gmail (or other HTML-accepting app).