Exactly. He didn't learn English (except maybe 2h per week in school) until adulthood (when he started in the UFC mostly). While the majority of people speaks english in Canada, the mix between French and English is not random. There is a huge territory named Quebec with cities where people speaks only French (except Montreal which is multi-cultural city). GSP comes from a small town called Saint-Isidore. Everywhere in Quebec, outside of Montreal, they have tv channel and movie in French, their school is in French, they used to have VHS movies only in French, they work in french and their friends are French, public services are in French and doctors speaks French... So one can have a nice life in St-Isidore without even learning some basic English.
Of course, if GSP was from Toronto and didn't speak english, he would be one weird individual! But having an accent in english when you are from Quebec is pretty much the norm!