To all those Valve Gift Grab achievement not registering on Steam.

First, it worked for me but I am not sure it would work for everyone.

It wasn't registering for me(I got the achievement on 16 December) then I restarted Steam. I let it load its store page, I clicked on the Great Gift Pile thing. Then I checked my inventory using the shortcut there and when hit the back button on Steam browser, it registered and I got a coal for it. Hope this helps for you too.