I tried the following
new GTileLayer(null, _minZoom, _maxZoom, { isPng: true, opacity: 0.6 });
But in IE8, while the alpha opacity works, the pixels in the PNG that already had translucence they show a grey color and standard opacity.
I have noticed also that PNG files that contain alpha transparency and their transparency is not modified by HTML style settings behave well in IE8, the problem occurs only when both PNG alpha transparency and HTML alpha transparency are applied to the same image.