Like others said GFO is just another way to export PO4 not a magic bullet. What I've found to be the trick is like any other filter it has to be "tuned" to your system. If it's lowering it much reduce the amount of media you're using or slow down the flow through that filter. It's just another tool that's available to do a job, IMO I like it a little better than growing chaeto if the reason for growing it is PO4 reduction since it's easier to control how much is "scrubbed" by the GFO vs the chaeto is going to grow or crash depending on other factors. Chaeto does have a ton of other benefits but IMO in a reducing PO4 perspective it's not as reliable as GFO. I run GFO to reduce the algae that grows on the glass and it's super effective at that, it's also how I know whether there's enough or not, I try to find and keep that balance. Depending on the GFO I can go 3 days or a week between needing to clean the glass, I try to keep it in the 5 day range.
The real trick is finding out where the phosphates are coming from and do what you can to fix that, biggest culprit I know of is normally the food you're feeding or the amount of food your feeding.