Why can’t I access my CBC Gem subscription from the US?

Ask CBC Gem.

Apple has nothing to do with their service.

Since it’s a Canadian company, they probably don’t allow access outside of Canada.

Just looked at the site for CBC Gem.

It includes content that other Streaming services have the distribution rights for outside of Canada.

Such as Justified, here in the states that is on Hulu/Disney +

Apple Media Services Terms and Conditions - Legal - Apple Media Services - Apple

- A. ”Our Services are available for your use in your country of residence (“Home Country”). By creating an account for use of the Services in a particular country or territory you are specifying it as your Home Country.

- J. “Certain Services and Content available to you in your Home Country may not be available to you when traveling outside of your Home Country."