Can I make my Gadgets opacity 0% ?

If you are still looking for a way to do this, see this article by Jack Wallen on TechRepublic:

How do I make Windows 7 desktop gadgets visible on mouse-over only? | Microsoft Windows | TechRepublic.com

It provides all necessary steps and can be done in a couple of minutes. If this is being done for multiple gadgets, make sure that you reset the GadgetOpacity to "1" for each gadget in the settings.ini file.

function setGadgetOpacity(opacity)
{
System.Gadget.Settings.write("PrivateSetting_GadgetOpacity", opacity);
}

call that function from anywhere in the gadget javascript file
setGadgetOpacity('0');