What do you call the grandson of your great uncle?

Answer 1

Your uncle's father is also the father of one of your parents and thus your grandfather. You are his grandchild.

If your grandfather has only one grand child, that person is you.

So your uncle's father's only grandchild is you, yourself.

Answer 2

The grandson of your great uncle is your second cousin.

Answer 3

or 3rd cousin.