"courageously," 1861, from game (adj.2) + -ly (2). In Old English and Middle English the adverb meant "artfully; joyfully."
Entries linking to gamely
game adj.2
"ready for action, unafraid, and up to the task;" probably literally "spirited as a game-cock," 1725, from game-cock "bird bred for fighting" (1670s), from game (n.) in the "sport, amusement" sense. Middle English adjectives gamesome, gamelich meant "joyful, playful, sportive."
-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.