There are certain shops where you can't use gift cards. The magic shop and the crystal shop are two that I know of off the top of my head.
There are a lot of threads about getting discounted gift cards. Many people first buy Target gift cards on a site such as Cardpool or Raise, then use those Target gift cards to buy Disney gift cards on Target.com. Without much effort, you get about 11% off. Once I get the Disney gift cards, I combine them as suggested above.
I pretty much just use gift cards when I'm in the park. Every time I buy something, I think "I just saved 11%!". And if I'm somewhere where there is an AP discount, I get 10% off...then pay with my gift card...so I'm up to 20% off on food, souvenirs, etc. Not bad, right?