Does flipping someone off count as a wave? If it does then he was fine. If not then yeah - a-hole status for sure.
I would have yelled, "It's called guttering B****!"
Nothing wrong with what you did - as you already know. That said I have found I have no patience for people that treat a group ride like the only race of the week anymore (meaning the other guy). Ride competitively, do things like guttering, etc....that's what group rides are great for, but it's uncalled for to act like he did. Tell him to STFU and pay for a number on the weekend before complaining about anything.