Gradation is to blend between colours or hues. The results can be varied, though many are aware of this technique through gradients in Adobe Photoshop, which is one of the most frequently used filters.
Paul Klee was amongst the most experimental of artists of the 20th century. Gradation was just one of many techniques that he made use of during his distinguished career.
Blending, shading, cross-hatching and hatching are techniques used for producing gradients between colours.
In Crystal Gradation there are fairly abrupt transitions between shades, leaving a pretty pattern of shapes. There is a suggestion at a three dimensional form to the crystals.
The large blocks of gradient also suggest animation frames of a cartoon, with too few frames to smoothen the movie as elements move around.