Great Wall of China vs mainland China

Great Wall of China and mainland China both are nouns.

Word	Noun	Adjective	Verb	Adverb
Great Wall of China	Yes	No	No	No
mainland China	Yes	No	No	No

Great Wall of China (noun)	mainland China (noun)
a fortification 1,500 miles long built across northern China in the 3rd century BC; it averages 6 meters in width	a communist nation that covers a vast territory in eastern Asia; the most populous country in the world