Unfortunately the USAians are so dominant in the region of the Americas that they've coopted the term American for most people. My Columbian friend hates when we refer to USAians as Americans because he says "hey we were here first" π. But unfortunately that's the way it is.
Yanks or Yankee Doodles is what we used to call them but they get rather upset these days when you call them that. I wouldn't call them gringos because it just sounds unnatural for a Brit to say that seriously.
Seppo, septic tank, yank. Love it! Cockney rhyming slang strikes again?