Just wondering about this. The email we got indicated a 50% discount at the store, but didn't mention any restrictions. When trying to use it, I get an error message saying that it won't work for the current cart/checkout. I can understand if that's by design, but couldn't tell for sure because the e-mail didn't specify that it couldn't be used for GK1:20th.
That code is actually for every other product in our store in honor of GK's release. In hindsight, we probably should have named it a bit differently.