If it was called Newtonity someone would ask why not Gravity.

Googling says gravity comes:

directly from Latin gravitatem (nominative gravitas) “weight, heaviness, pressure,” from gravis “heavy” (from PIE root *gwere- (1) “heavy”). The scientific sense of “downward acceleration of terrestrial bodies due to gravitation of the Earth” first recorded 1620s.

https://www.etymonline.com/word/gravity

When in doubt, blame Latin.