WordNet sense 1 (marked with or divided into degrees):

calibrated

WordNet sense 2 ((of taxes) adjusted so that the rate decreases as the amount of income increases):

regressive

WordNet sense 3 ((of taxes) increasing as the amount taxed increases):

proportional

WordNet sense 4 ((of taxes) adjusted so that the rate increases as the amount of income increases):

progressive

WordNet sense 5 (taking place by degrees):

gradational, gradatory

WordNet sense 6 (proceeding in small stages):

gradual