Philadelphia. 

 I was told by a person from Latin America that he used the word "gringo" in protests against US military personnel in Latin America during times of governmental upheaval.  He said that the US military wore green, so the Spanish-speaking protestors would yell "Green Go!" to make their protestations understood.  He said that because of this the troops became known as gringos, and the term came to include any person from the US, especially pushy people.