The only free and comprehensive online etymological dictionary of the Spanish language
giro (1) m. (Noun) "turn"
15th cent. From Vulgar Latin girus "circle," "turn." From Latin gyrus "circle." Borrowed from Ancient Greek γῦρος (gyros) 'id.' Probably from Proto-Indo-European *guH-ro- "curved," and therefore from the verb *geu̯- "to curve."
Italian giro

In the 3rd or 4th cent. text Appendix Probi we find the line: gyrus non girus "[the word for 'circle' is] gyrus, not girus." The author's spelling correction proves how the word was pronounced by common speakers, and offers a rare glimpse of the evolution of Latin into Romance languages such as Spanish.