gradation vs shading

gradation and shading both are nouns.

Word	Noun	Adjective	Verb	Adverb
gradation	Yes	No	No	No
shading	Yes	No	No	No

As nouns, shading is a hyponym of gradation; that is, shading is a word with a more specific, narrower meaning than gradation:

gradation: the act of arranging in grades
shading: a gradation involving small or imperceptible differences between grades

Other hyponyms of gradation include blending.

gradation (noun)	shading (noun)
the act of arranging in grades	a gradation involving small or imperceptible differences between grades
a degree of ablaut	graded markings that indicate light or shaded areas in a drawing or painting
relative position in a graded series