unfortunately this doesnt happen in Xperia Ray-- not in the Gmail App.
the rules for copying text in Xperia Ray are different for the gmail app and for the browser
i found them by hit and trial method now
in gmail app first do the long press thing as described above by DURX but then u have to double tap it - like what is known as double click on a pc- this will result in selected text to be copied on the clipboard. after that ofcourse paste as u feel like
in browser, we dont have to double tap to copy-- just longpressing on the selected text will do