It depends if it is a maternal or paternal great grandmother. I am not 100% sure of this and was hoping a native Mandarin speaker would answer this ( I am only a student) but here is what I found:If it's a paternal great grandmother (father's father's mother) it would be曾祖母 or in pinyin zeng1 zu3 mu3.If it's a maternal great grandmother (mother's father's mother) it would be : 曾外婆 or in pinyin zeng1 wai4 gong1.
