yournamehere

A Grandmaster is one who has attained the rank of 8th Dan, or 8th degree black belt (A Master rank is reached at 4th Dan). The rank of 10th dan is usually reserved for the founder, or highest-ranked living member of a particular style of any martial art.

Technically, though - I suppose if you break away and start "Joe's Karate Do", you can call yourself the Grandmaster of your own style.

This is one of the reasons for so much political in-fighting in the martial arts - because there is so much inconsistency regarding rank, styles, and status among the hundreds of federations, associations, and societies representing all of the different styles of martial arts.

Your results may vary.

__________________
If you want to avoid 95% of internet spelling errors:
"If your ridiculous pants are too loose, you're definitely going to lose them. Tell your two loser friends over there that they're going to lose theirs, too."
It won't hurt your fashion sense, either.