GAC stands for "Guest Assistance Card" which is a Disney aid for special needs visitors. There is a Frequently Asked Questions section here that will explain the GAC.
Fast Pass is a system Disney designed for it's guests to use their park passes and get a Fast Pass to return to a certain attraction at a time when they can by-pass the line. If you want to ride Space Mt. and there is a wait of 45 minutes, you can get a FastPass and return at the time shown on it and not have to wait in the 45 minute line. It will give you a window of time to come back and use the FastPass line.