The GCs will be accepted at most places - resort, restaurants, stores - but there are some places that won't so it's best to have some cash and an alternate form of payment just in case.
Cash only, for example, include balloon vendors, some souvenir vendors walking along parade routes and refreshment vendors in some of the longer queue lines and on the Boardwalk.
Some third party locations also do not accept Disney GCs, such as the restaurants and shops (except the Disney gift shop) in the Swan and Dolphin and Shades of Green.