Ah. We're a peculiar lot, as can be expected from people who describe chips, pizzas and hot dogs as "all-dressed". Come to think of it, my generation described both French fries and crisps as chips.
I think it may be that, although there's probably not that much to choose from as to ethnic mix between our countries, we were, in your great-aunt's generation, an overtly bicultural nation (not without tensions, as you know), and are now probably more overtly multicultural. (The key word here is "overtly".)
This leads to:
"Where are you from?"
...which would be a declaration of both Polish and Canadian culture.
For instance, in certain contexts, I'll call myself Scottish-Canadian. My family background is actually Scots, Irish, English and Mohawk, but, in my estimation, Scottish attitudes and culture have come down most clearly. (What I don't know about Mohawk culture could fill an encyclopaedia, possibly several - that's quite high up the family tree in my great-grandmother's branch. Great-grandfather, however, came directly from Ross-shire.) There is an element of picking and choosing one's cultural identity in all this, and we're generally good with this here.
Doesn't mean we aren't keenly Canadian; though.
Dunno about the origin of your family name's spelling. Both Gotreau and Gotreaux are legitimate spellings. French names can have variant spellings and pronunciations, sometimes quite a few. Favre, Favré, Faure and Fauré are all the same name, for instance. What I find interesting about the spelling of your name is that its French pronunciation is pretty much the same as Gotreau or Gotreaux - French doesn't have a theta sound.