1690s, "without cause or reason," from gratuitous + -ly (2). From 1716 as "without cost to the recipient."
Entries linking to gratuitously
gratuitous adj.
1650s, "freely bestowed," from Latin gratuitus "done without pay, spontaneous, voluntary," from gratus "pleasing, agreeable," from gratia "favor" (from suffixed form of PIE root *gwere- (2) "to favor"). Earlier was gratuital (1590s). Sense of "uncalled for, done without good reason" is first recorded 1690s.
-ly 2
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.