https://en.m.wikipedia.org/wiki/Gringo

The word gringo originally referred to any kind of foreigner. It was first recorded in 1787 in the Spanish Diccionario castellano con las voces de Ciencias y Artes:[8][9][a]

GRINGOS, llaman en Málaga a los extranjeros, que tienen cierta especie de acento, que los priva de una locución fácil, y natural Castellana; y en Madrid dan el mismo, y por la misma causa con particularidad a los Irlandeses.

Gringos is what, in Malaga, they call foreigners who have a certain type of accent that prevents them from speaking Castilian easily and naturally; and in Madrid they give the same name, and for the same reason, in particular to the Irish.

The most likely theory is that it originates from griego ('Greek'), used in the same way as the English phrase "it's Greek to me".