Honestly I don't know much about the whole GM title outside of my little fish bowl. However, in my association there are technically a few grandmasters, although NONE of them use that title except for our Kwan Jang Nim. Grandmaster Sok Ho Kang has raised many master instructors, and many of those master instructors have raised a few master instructors of their own. So technically I suppose my Kwan Jang Nim should be a Great Grand Master, but I no one uses such a term.
Seobahmnim and kwanjangnim are used more often than master and grand master in my schools. I think they are more clear on definition than master and grand master. After all there are a couple different definitions of master and grandmaster. Some say that you can only be a grandmaster at a certain rank, some say it just means you are a master that has raised another master.
It's all semantics to me, and I don't particularly care what people call each other, as long as it is out of proper respect.