Disney Gift Cards can be used in most locations in both parks, except at some (maybe all) of the third party shops in the parks. They aren't accepted at Ghiradelli's or the Arribas Brothers shops (those are the ones I've tried - I think there might be another shop, but I can't remember) and they can't be used for penny press machines or balloon sellers (both are cash only).
In DTD, Disney Gift Cards can be used at any of the Disney-owned shops (WoD, Pin Traders, Marcelines, D-Street, Vault 28, Anna & Elsa's Boutique, etc.), but not at the third party locations.