Your grandfather's first cousin is your first cousin twice removed. (He is not your grand-uncle because he is not your grandfather's brother or brother-in-law.)
First cousins twice removed
"A person shares a first cousins twice removed relationship with their grandparents' cousins and their cousins' grandchildren. At least one set of one person's grandparents are the great-great-grandparents of the other person."
You must check out this link; it's so comprehensive and provides a relationship chart and a family tree with examples of many types of cousins.
Keep in mind that some people (many, I think) do not understand this "removed" business and would simply refer to your grandfather's cousin as your "third cousin" even though you are not on the same level of your family tree (as first, second, and third cousins are); you're two generations apart (thus "twice removed"). They are misinterpreting a loose definition of third cousin such as this:
"A person whose closest common ancestor with another person is a great-great-grandparent" [https://en.wiktionary.org/wiki/third_cousin ] versus going by a more precise definition such as "a child of one's parent's second cousin" [Google Dictionary].