I want to increase or decrease the transparency of gnome-terminal in Unity.
On the other hand you can try CompizConfig. When you start it go to "Accessibility" and activate "Brightness, Opacity and Saturation" plugin. When you open it on Opacity tab you can set combination for mouse buttons or keyboard keys to change opacity of any active (focused) window. Default in my case was "Alt"+Scroll wheel. This is working perfectly for me. Normally I have no opacity set but when I need it I just press "Alt" and with a scroll whell adjust opacity to my desire. Hope this will help.