YOU SAID:

My grandpa's grandpa is my grandpa's grandpa's grandpa's grandson.

INTO JAPANESE

私のおじいちゃんのおじいさんは私のおじいさんのおじいさんのおじいさんです。

BACK INTO ENGLISH

My grandfather's grandfather is my grandfather's grandfather's grandfather.

INTO JAPANESE

私の祖父の祖父は、私の祖父の祖父の祖父です。

BACK INTO ENGLISH

My grandfather's grandfather is my grandfather's grandfather's grandfather.