Yes, your GPS will work in Canada. If you have OnStar, the question you should be asking is will OnStar work in Canada? Yes, but you should power your Volt off and then restart the Volt shortly after you enter Canada to ensure that your 4G LTE modem connects to AT&T's Canada network.
No need to power off the car to connect. Your can can move from network to network in the US, why on earth would you have to power off when you get to Canada?
Unless it's to get in line for the team of dog sleds that will take you the final few miles to your destination...
On that note:
A few years ago my cousin, who grew up in Buffalo, was living in his new home in Florida. His new wife whom he met there had never left the state and they were planning a trip to Canada.
He had her convinced that when they got to Canada they would have to park in a huge parking lot and take dog sleds for the remainder of their trip.
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