gradually (adv.)

early 15c., "having steps or ridges," from Medieval Latin gradualis, from Latin gradus "a step; a step climbed; a step toward something, a degree of something rising by stages" (from PIE root *ghredh- "to walk, go"). Meaning "arranged by degrees" is from 1540s; that of "taking place by degrees" is from 1690s.

common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).

Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.