gradualyoudaoicibaDictYouDict[gradual 词源字典]
gradual: [16] Latin grādus ‘step’ has been a remarkably prolific source of English words. Beside grade [16] itself, it has contributed the derivatives gradation [16], gradient [19], gradual (from medieval Latin graduālis, literally ‘proceeding by steps’), graduate [15], and retrograde [14]. The derived verb gradī ‘walk, go’ has produced ingredient, and its past participial stem gress- has given, among others, aggression, congress, digress [16], progress [15], and transgress [16].

And degrade and degree are of the same parentage, the latter filtered through Old French. The origins of Latin grādus itself are not known.
=> aggression, congress, degrade, degree, digress, grade, gradient, ingredient, progress, transgress[gradual etymology, gradual origin, 英语词源]
gradual (adj.)youdaoicibaDictYouDict
early 15c., "having steps or ridges," from Medieval Latin gradualis, from Latin gradus "step" (see grade (n.)). Meaning "arranged by degrees" is from 1540s; that of "taking place by degrees" is from 1690s.