If you look in post #6 of the disABILITIES FAQs thread, you will find the answer to all of your questions about Guest Assistance Cards.
The GACs are dated for the specific time of your trip, so you WILL need to get a new GAC this trip.
The assistance at each attraction varies depending on the type of attraction, what you need, what is available, how busy it is and even things like staffing . You may get different treatment at the same attraction, even on the same day because those things can vary.
Many people find that things that help you know what parks are busiest when are more helpful than a GAC. The GAC can help only for attractions, but things like a touring plan or TourGuide Mike can help you avoid the busiest places at the busiest times. Fastpass is also helpful because when you return at the time printed on your Fastpass return card, you will usually be waiting 10-15 minutes. IF you need a wheelchair accessible ride car, your wait can sometime be longer because you may have to wait for a wheelchair accessible ride car or may have other people with disabilities ahead of you.