Herbivores are important just as the proper balance of snails and hermit crabs. It is also going to require you to manually remove what you can over the course of a couple days. Grab a cup of water to rinse your hands in and leave the GHA. Basically what you are doing is going to glove up, safely reach in the tank, pinch a tuff of GHA, gently pull and twist. Lift out of the tank, place in cup or bowl of water, rinse and repeat for about 20 minutes. Come back later and do it again. And again. And again. Couple days you should have made a pretty good dent in it and can now look at other options.