A good ol' yellow foxface will usually mow GHA as well as bryopsis better than any other animal that I've found.
I do agree that tackling the nutrient problem is important, but honestly, even if you get the system at zero PO4 and NO3 it could take months for the algae to dissipate.
Secondly, if you do the GFO route, work up the GFO amount over a few months. If you just add a few cups and drop the PO4 to zero, you'll shock the crap out of the system, and most likely kill a bunch of coral in the process.