I’d never have guessed that one. I figured Urban Dictionary would have it listed but that particular abbreviation is obscure enough that it’s only listed on the second page of results: https://www.urbandictionary.com/define.php?term=gtk
18 and 88 are the extent of my awareness of far-right codes/dog-whistles.
It means that to a vanishingly small number of quite deranged people. Hand over your language to them if you so wish. I won't be doing so.