Giant Tiger has a page with regional restrictions, and it doesn't mention anything about returns being limited, so I would assume returns are a go. But if you phone they can confirm that before venturing out.
https://www.gianttiger.com/regional-restrictions (I also want to point out that GT has a really awesome return policy that I don't think has a time limit, and we're not supposed to be making essential trips. So if it's something that can wait I would encourage you to do that.)
I can't find anything about Wal-Mart either way, but they don't have enough manned tills to process everyone even with the restrictions, so imagine they are open in at least some limited capacity (maybe every other one open?). They are likely wiped down between customers.