I, of course, respect Michael's feelings, but personally, to me, gringo is a rather mild word. I spent a lifetime near the Texas border and grew up being called a pepper-belly, taco, greaser and bean. The moral of this is never insult anyone, they may grow up and turn out to be a writer. Good luck on the success of your book. However, I disagree that one word is stronger than another.
Words mean different things to different people, depending upon their experience.
Today, the word has lost its sting and is used by many as an endearing term to denote a person who overcame a crisis. The best course of action is don't use offensive words because someone will take offense. The only devils in the world are those running in our own hearts. That is where the battle should be fought.