You mean just to copy text in a new e-mail? I don't know if you're familiar with the use of Ctrl + C and Ctrl + V..?
Ctrl + C copies something that you have selected to a virtual clipboard, Ctrl + V pastes something that you have copied before from a virtual to the place where your cursor is located.
When you have copied something and would like to paste this in Gmail, just press Ctrl+V at the location where you want the text/object to appear.
(If this is not what you mean, please reformulate your question..)