I'm going to disagree with the previous posters. If the service you received does not warrant the gratuity that is applied, ask to see the manager, explain what went wrong, and tell him or her what gratuity you believe to be appropriate. A "gratuity" is not and cannot be mandatory. A "service charge" could be, but I don't believe they're calling this a service charge.
As a former bartender, I have no interest in stiffing any waiter, and usually tip at or about 20% for good service. But I did have a situation not too long ago (not in WDW) where I was paying for a large party (12 or 14 of us, so the gratuity was automatically applied), the service was terrible, and I advised the manager what our problems were and that my 10% tip was all the waiter was going to get. He was very understanding and I had no problems with it. (I don't think I've EVER not left anything for a waiter or waitress.)
That said, I understand this is one of those topics that seems to arouse the passions of many, and about which reasonable people can disagree strongly. So I'm prepared to be flambeed and fricasseed . . . .