Catharine i read this awhile ago from several reefers and I never tried it until last month and I can confirm that this undisputedly worked for me.
I had 5 patches of GHA that would not go away. My rabbitfish stopped eating it as soon as it learned that nori tasted much better. So I finally tried this method which is applying hot boiling water directly onto the GHA. Initiallly I tried doing it with a turkey baster several times before and didn't work well because the hot gases in the baster pushed out the hot water as soon as i sucked it up and lifted it out of the hot water container.
I finally got one of the flexible rubbermaid containers that looks like this:
I removed the white straw tab on the cap and you will see a hole on the cap that lets the liquid out into the white straw tab. This hole will fit a 1/4" flexible hose very snugly. Insert a hose that is long enough to manuever under and around rocks.
Turn off all tank water circulation devices.
Pour hot boiling scalding hot water into the bottle, cap it up,
Turn the bottle upside down into your tank, and squirt the hot water all over a good sized patch of GHA.
Squirt slowly so the hot water "lingers" on the GHA.
Depending on your tank size, I would consider target one rock at a time. Continue doing this until you've targeted all your rocks and then start again from the beginning. You will start seeting GHA shedding off the next day so make sure you clean the powerhead intakes or overflow intakes.
Keep your skimmer skimming WET! this will ensure the dying GHA doesn't fuel more nutrients into your tank.
If you have corals nearby, move them before doing this if you are concerned. My acros didn't seem to care or mind when I did it when them nearby.
Some people will argue that this will kill your liverock's bacteria. I will argue that it will kill your GHA and some bacteria on the surface under the GHA but your liverock's inside bacteria will be fine.
Similarly, this product could work too but harder to apply under rocks without the hose attachment.
Seriously, try this method first. If you are apprehensive, do it to one or two rocks first. My GHA patches disappeared after a week and I am currently GHA free.