No supermarket is particularly better than another. The actual store stock varies more from individual store to store though. Basically all the supermarkets in Ontario are owned by the same two corporations but with different branding on the individual stores. They seem to stock differently though even if they share the same branding.

The gluten-free labeling rules for packaged foods are not different from in the US in any significant way. The allergen rules are a tiny bit different but not the gluten rules.

I have found restaurant food is very risky. In the US up to over half of gluten-free restaurant food tests at >20 ppm gluten but it may be worse here.

I called every single gluten-free dedicated restaurant in Toronto and they all volunteered right away that they were not celiac safe and that the "gluten-free food" is not intended for celiacs. None of the chains are safe. Most have disclaimers in the small print on their websites.

In Toronto there are several regular restaurants that actually have celiac-safe gluten-free food. I think the closer the actual proprietor is to the kitchen probably the safer it is.

The GF bakeries are all safe though and Starbucks often has packed GF brownies, cookies, or rice krispy treats.