Based on Steven Ogg, Trevor's voice actor and the fact that Trevor says he grew up on the "Canadian border region of America" it can be implied Trevor was brought up in the Alberta province of Canada near Montana.
While he rarely speaks about it, the protagonist Trevor Philips was born and raised in Canada, and is quite self-conscious and insecure about his upbringings. He gets quite upset when people comment on his apparent accent. This is what caused several of his rampages after being mocked and insulted by the various residents of San Andreas, including the Rednecks, Vagos, Ballas, hipsters, and even the military.
On Jockcranley.com when the player scores bad on the American Patriotism test a message appears reading "If you hate America so much go live in France, part of it is in Canada. Or vote for Jock Cranley and turn your life around now", a reference to the French-speaking province of Quebec and spoofing the blinkered world views of Conservatives.
In GTA Online, Merryweather's website lists Canada as "Like Australia but with snow", which suggests that Merryweather may operate in Canada alongside the Canadian Armed Forces, and various Canadian police agencies, such as the RCMP.
Weazel News mentions a pipe line from Canada to Los Santos, sending Canadian water to LS, since water in Los Santos is apparently polluted.
The Elitás Travelwebsite states that the Maverick helicopter is manufactured in Canada, this might be a reference to the Bell 240B which is also manufactured in Canada.
The Marshall, which is exclusive for players who have played GTA V on the original PS3 or Xbox 360 version and played it again on PlayStation 4, Xbox One or PC version, can sport a Canadian livery (red and white colour scheme, with a Canadian flag in the back)