Another person grandparents
祖父母　My grandparents
山田さんの祖父母
山田さんのご祖父母
Which is the right one ?
Or does exist another word to say another person grandparents ?

Question

Another person grandparents 
祖父母　My grandparents 
山田さんの祖父母
山田さんのご祖父母
Which is the right one ?
Or does exist another word to say another person grandparents ?

Answer

Both are okay.
「ご」is a word to make the words politer.

and 
ご祖父母／祖父母are fine to use for this situation, too.
There are no special ways to call other person's grand parents.

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