The right-button context menu in gnome-terminal brings up the popup about half the time, in my experience. The other half of the time it tries to select text, with no clear way to switch between these two "modes." It looks like a bug to me.
Right-click extends the selection if you've selected any text since the last
time you typed in the terminal. You can get the menu at those times by holding
down the control key when you right-click.