shading vs gradation

shading and gradation both are nouns.

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

As nouns, gradation is a hypernym of shading; that is, gradation is a word with a broader meaning than shading:

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

Other hypernyms of shading include graduation.

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