Both GCM (for Visa cards) and Giftcards.com are gone from shopping portals. The Cashback Monitor page for Giftcards.com suggests this is only temporary, but only time will tell if it does actually come back.
I find it interesting both sites would be removed at the same time. They are owned by the same company, so there’s likely an company-wide change going on. Change is seldom welcomed within the points and miles community. My speculation (and remember it’s just speculation) is it’ll be negative, especially considering how many people reported difficulties tracking Giftcards.com purchases.
Nothing to do besides waiting to see, though.