All gradient filters, whether made of glass or plastic, are coated filters. As far as I know, there are no gradients in the glass. Is due to the production of glass. Nevertheless, there are good to very good results with the glass filters. Lee filters are usually made of plastic, with the ordinal number CR39, from which glasses are also made. They are also coated. In fact, they are very good. Due to this complex production, the filters cost a lot of money. And at Lee Filters they are not cheap especially, although they are made out of "plastic", except for the IR filters. They're made of glass. 

I believe this camera includes an option in the camera settings for the red filter used for underwater. I don't believe the external filter would be needed if this is turned on. 

no color cast 

Yes. 

The best combination of nano-film stacking makes the overall spectrum smooth and ensures uniform color transmission into the lens in the visible band, resulting in high-fidelity images;