Window.Opacity (gb.qt4)

This property has an effect only if you run a compositing window manager.

Using a compositor does not guarantee this property will work as compositors do not all work the same. Gambas can set the property but if it will work depends on the x-window system you use. For example on LinuxMint-20 testing showed it will ONLY work if you use compiz with compositing enabled. but not with metacity or marco.
On Debian-10 it works with compiz and marco with compton but not marco with adaptive compositing.
You should consider this property a possible feature for your application and not a needed requirement.