I personally think GFO is about as perfect a phosphate remover as you will find. The problem with GFO is it works too well, and when run properly in a reactor, tumbling correctly, it will completely remove phosphate. It is becoming more and more evident that corals require some phosphate and nitrate. So removing all of it can cause your system to crash.
Deven from Reef Dudes recommends that you run a very small amount of GFO and only run it temporarily to fix the problem you are having. Then get it off the system to avoid the potential for a crash.
I have personally used it incorrectly and packed a reactor chamber full of it and it did pretty much nothing. If it’s not tumbling, it won’t do the job you intend for it to do, and will just bind up and be pretty worthless. So if you do decide to use it, use less than you think you need, and only run it for a limited time.
GFO is also very expensive and I think paying such a high price for premium rust is a bit ridiculous honestly. However, it is a tool in the toolbox, and should be used only when that tool is necessary.