Try this address:
Walt Disney World Guest Communications
P.O.Box 10,000
Lake Buena Vista, FL 32830-1000
I have received courteous replies from letters I have sent to that address in the past (not necessarily complaints, mind you); other people tell me that Disney is not quick to respond to complaints. I would also write to the Grand Floridian directly.
I'm just curious - would you mind sharing your story? Two sets of friends of ours have spent a week there in the past year, and one couple had the time of their lives - unfortunately, the other couple HATED the Grand Flordian!