I've run across plenty of metal, G, GA, GB and GC, and of course their numbered cousins, never heard of or seen a 6L6GT. Google finds me pics of some Japanese TEN branded ones, but no old US production.
I know there is a GT sized Bendix tube thats 6L6 class, but internally it looks like a pair of 6V6 jammed into one bottle. Can't remember the number but its definitely a weirdo.