What nouns are Grandfather Paul?

The term 'Grandfather Paul' is a compound, proper noun, a Word for a specific person.

The noun 'grandfather' is a compound noun on its own, a combination of the adjective 'grand' and the noun 'father', combined to form a Word with its own meaning.

A proper noun is the name of a specific person, place, or thing. The noun 'Paul' is a proper noun as the name of a specific person. The common noun 'grandfather' is a proper noun as a name you call a specific person.