One of the most annoying side-effects of using a rich text editor frequently (like the Gmail composer) is that the copied text retains it formatting when you paste it. Most people copy the text to a simple editor like leafpad, copy it again and then paste it to the rich text editor. Some even go greater lengths and install a clipboard manager like Parcellite.
On the latest versions of Ubuntu you can paste text sans the formatting using the keyboard shortcut
<Ctrl-Shift-v>. Tested it on Ubuntu 14.04. Works like a charm!
Select text and middle click to paste does the job too.