In Taekwon-do, a Grandmaster is a practitioner who has attained the rank of 9th Degree Black Belt (also known as 9th Dan, for some). He is called the "Sasung-nim".
A Master is one who is 7th or 8th Degree Black Belt. Ranks below that are commonly known as "Sabum-nim" (Instructor) or "Boo-Sabum-nim" (Asst. Instructor).
Whether or not a person should be deemed a Grandmaster, is another matter. I know people who promote themselves to the title through shortcuts. This is very unfair to the others who worked hard. Likewise I know some old timers who were involved in the art since many years ago and their level of knowledge and skill deserves the title.