My GC account is registered in the US, however in my mobile GC app (on Android), it simply states that the Canadian Radar is provided by Environment Canada. This, by the way, is under Settings/Weather.
You can also see the data from any Weather Underground station in CIQ with this one. Be it your own back yard or one that's closest to your current location.
https://apps.garmin.com/en-US/apps/f509fc4e-7f1f-48a5-b337-ed86668c5f6c
The native widget doesn't provide a "source" or a timestamp for how old the data is. This one does... And it ONLY need GPS when you tell it to change locations and then fires it up on the watch to get the location. No mystery.....
This answer is obviously incorrect.
In the US, Garmin gets its weather data from National Weather Service Forecast Office . weather.gov
Unfortunately the data is not very accurate and the weather station could be several miles from your location.