The Spanish word 'gringa' is the feminine form of 'gringo', which has an interesting etymology tracing back to the Latin word 'graecus' meaning 'Greek'. This evolved into the Spanish word 'griego', which meant both 'Greek' and 'unintelligible speech' (similar to how English speakers might say 'it's all Greek to me' when they don't understand something). Eventually, 'griego' evolved into 'gringo', which came to specifically refer to foreign people, especially English-speaking foreigners. 'Gringa' is simply the feminine form of this word.

A simpler related Spanish word is 'griego', meaning 'Greek'. While 'griego' is used literally to refer to things from Greece, it's interesting to note how its metaphorical use for 'unintelligible speech' led to the development of 'gringo/gringa' as terms for foreigners.

While not directly cognate, English speakers might find it interesting that the concept behind 'gringa' has a parallel in the English phrase 'it's all Greek to me', both stemming from the historical use of 'Greek' as a metaphor for something foreign or unintelligible. The English word 'Greek' itself comes from the same Latin root 'graecus' that gave rise to 'gringa', though through a different path.