Texan here. Yankee is definitely not a neutral word to refer to everyone from the USA. Some people down here will fight you over it, but most would just give you a confused look.
I’ve always understood gringo to mean white person, especially one who can’t speak Spanish. The term is sometimes used in Mexican restaurants to let the staff know that you can’t deal with too many jalapeños.
Someone who will treat you well won’t need to tell you that they will treat you well. It’s kind of like how liars and scammers make a big deal about how honest they are. Another big clue is that he was telling you this while both of you were with other people. He cheats on one partner, he’d cheat on you, too.