What approach are you looking at? All approaches in Canadian Southern Domestic Airspace, including GPS approaches, show directions relative to Magnetic North regardless of lattitude. In Northern Domestic Airspace, which has no major airports, all directions are in True regardless of type of approach GPS or otherwise. No references to magnetic North and you would need to set your DG to true heading and your Garmin to true. I don't know of any navigation system that automatically switches from Mag to True without pilot input. The Garmin has three user selectable modes: True, Magnetic Auto, and Magnetic User.
High lattitudes is not a problem for magnetic compass systems, being close to the magnetic North pole is. This is why high lattitude plates outside of the area of compass unreliability (in otherwords not close to the Magnetic North Pole) still use Magnetic North. High lattitudes are a problem with both magnetic and true headings rapidly changing as you follow a straight line, which is why they use grid reference.