There are grains other than wheat that contain gluten, including barley and rye, so being wheat-free doesn't mean being gluten-free. And many grains are processed in facilities which process wheat, so even if you get oats or rice (which are gluten-free in and of themselves), if they've been cross-contaminated with the wheat, they may contain traces of gluten, in which case the processors have to report that. It's like when something is processed in a facility that processes nuts, they have to say it *may* contain nuts or traces of nuts, even if it might not, so you have to be super-careful to avoid any cross-contamination if you know you or your LO is super-sensitive or highly-allergic to something that *might* be in there.
As to what needs to be reported, in the States they have to disclose certain things like nuts, dairy, soy, and wheat and eggs, but I don't know what needs to be reported in Canada. I imagine it's mostly the same stuff, though.