Grandpa/ma + First Name is what we used, dropping the first name when there was no possibility of confusion in context. I've also seen + Last Name.

One gets the impression from pop culture that it's more common to have a designated opaque term, e.g. if your father is Dad, your grandfather is Pop, and your other grandfather is some other synonym. But I never encountered this among other children in my childhood.

Nor, despite our being Jewish, did anybody in the community use the Yiddish terms like Bubbe.