1610s, "pertaining to giants," from Latin gigant- stem of gigas "giant" (see giant) + -ic. Replaced earlier gigantine (c. 1600), gigantical (c. 1600), giantlike (1570s). The Latin adjective was giganteus. Of material or immaterial things, actions, etc., "of extraordinary sine or proportions," by 1797.
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.
<a href="https://www.etymonline.com/word/gigantism">Etymology of gigantism by etymonline</a>
Harper, D. (n.d.). Etymology of gigantism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/gigantism