I tried the following
new GTileLayer(null, _minZoom, _maxZoom, { isPng: true, opacity: 0.6 });
But in IE8, while the alpha opacity works, t
It's definitely a well-known bug - see here, here, and here. Just to throw a few links:
And then there's this addressing a probably different bug in an early release of IE8. He got it solved by enlarging the image - maybe worth a try if you're working with a 1x1 image.
Maybe you can work around this by using different pngs with different opacities, and no CSS opacity?