GHA is the biggest headache a reefer can face in this hobby. Many get so disappointed that they simply leave the hobby. However, many has successfully overcome such a problem and end with beautiful tank.
I have fought GHA in my 92 g tank for years and finally my tank is looking better than before. I still have GHA in there but they are under control and only in a few spots.
GHA lives on nutrients like PO4 and NO3 which come mostly from the rocks and from feedings. Those coming from the rocks may take months to leach out. It is very important that you don't overfeed the tank, especially with frozen food which is very high in PO4.
The game plan now is to export the PO4 as aggressively as possible with frequent GFO changes, small feedings with flakes or pellets, and weekly water change. As a last resort, take the worst affected rocks out and dip into 3% 50:50 H2O2 for 3 min. Good luck!