Can I add a colorbar in kml image?

I need to visualize grd images on google earth with different color scales. I can use grd2kml to view the image on google earth but prefer a way also show the color scale. Any way to do it?

Also, is there a way to make kmz instead of kml + png?

There is no option to include a colorbar using grd2kml. You could open a feature request or create the colorbar using colorbar (in modern mode) or pscale (in classic mode) to import into google earth separately.