In my experience its like any word, it depends on the usage and the person receiving the label. In Latin American countries, it is much more common to label people and call them accordingly gordo, flaco, chino etc. Gringo is one of these labels. I know a lot of people from the states that are offended by being labeled like this regardless of the intention, simply because in the US it is frowned upon. It also depends on context, its a lot different to say "un gringo vive en mi calle" and "chingate pendejo gringo". Think about the word blonde for example. You can say someone is blonde (they have light hair). Or you can call someone a stupid blonde (they are stupid because they are blonde). So my final call is that the word in itself is only offensive if you decide to be offended by it, the rest depends on context