"Gringo" was already in Spanish in the late 18th c. Whether the origin was 'griego' or not is uncertain. The dictionary of the Royal Spanish Academy does not speculate on its origin.

Diccionario de la Lengua Española (Royal Spanish Academy) gives its common meanings, a few below:
1. Foreigner (extranjero), especially an English-speaker (add French for Mexico).
2. A speaker of English (found in Central America; Honduras, Nicaragua, Cuba, El Salvador)
6. One with a light, blonde or ruddy complexion (Bolivia, Peru, Honduras, etc.)
7. One whose language is not understandable (Colloquial, widespread).