Yes, otherwise you are just saying the person is a wonderful, i.e. great, grandmother. The hyphen denotes relationship.
Some say yes, you do need a hyphen because the term great is integral to the definition of great-grandmother. Otherwise, a great grandmother is just a grandmother who is terrific. Other people see no need and d not use hyphens for this.
Your grandmother's grandmother is your great-great-grandmother.
Your great grandmother's mother is your great great grandmother.
Tatarabuela Grandmother: abuela Great grandmother: bisabuela Great great grandmother: tatarabuela
Your father's grandmother is typically referred to as your great-grandmother.
You add another "great" for each generation back in history. So the mother of your great great great grandmother is your great great great great grandmother.
your great great grandmother.
Your great grandmother's brother is your great great uncle.
You grandmother's father is your great grandfather and his wife (your grandmother's mother) is your great grandmother.
Ruth was the great grandmother of King David.
Your great grandmother's sister is your great-great-aunt.
The singular form of great-grandmother is great-grandmother.