A gradient copolymer comprises or consists of n polymer components. The n polymer components each independently represents an addition polymer of a monomer of the formula (I), and/or a mixture thereof, or the n polymer components each independently comprises or consists essential of one or more structural units represented by the formula (I-1), The symbol n represents an integer within the closed interval [5, ∞]. The average side chain carbon number of the i-th polymer component as determined according to the nuclear magnetic resonance method is expressed as Xi, with the symbol i representing an arbitrary integer from 1 to n, the relationship X1