But.... this issue is not GDA specific, it's algae and plant specific.
Simply directing lighting elsewhere is a rather obvious notion.

I am not sure why more lighting companies do not do such obvious things for planted and reef tanks.

Having an angled spread vs the tank H and W is a rather simple thing.
Not having direct light hitting the front, sides and rear would be ideal.

But they'd rather rattle on and on about how their "special heavily researched spectrum prevents algae".

They can go suck eggs for all I care.

You can place a screen, light filter, dark plastic etc in front of the glass easily enough and see how light influences the GDA. You can do this on the side so it does not prevent the viewing etc as well. Or all the glass sides etc.

This in itself might be a cure for GDA also. Simply cover the glass, or use plastic/glass etc that will get covered with the GDA instead of the glass behind it, then remove it along with the GDA.

Regards,
Tom Barr