Hi Barbara,
To obtain a GAC, go to Guest Relations in any of the theme parks. All Guest Relations locations are located near the front entrance inside the parks. You will only need to visit once to obtain a GAC which they can date to be valid through the end of your stay. My son has CP & uses a larger special needs stroller instead of a wheelchair. If your child is in a stroller, also ask for a tag that will indicate to Cast Members that he/she uses a stroller instead of wheelchair so that you will be allowed to take the stroller in attractions that accommodates guests in wheelchairs - otherwise you'll be asked to park the stroller while you're on an attraction.
I have found Disney to be super accommodating with my son. They have a section on their website devoted to traveling with guests with disabilities - click
here. The Guide Maps in the theme parks will indicate which attractions guests may remain in their wheelchairs & which ones they will be required to be mobile in order to enter the attraction. You can click on the link in the disabilities section for Mobility Disabilities to plan ahead.
I know that a lot of people with CP also have seizure disorders. Please be aware that some attractions at WDW do use strobe effects that can trigger seizures in some patients. If your child is sensitive to strobe effects, you should probably avoid them. I'd also recommend traveling with a medical history for your child - just in case. You know your child's limitations so use your judgment when deciding which attractions to do. While your child may not be able to experience every attraction at WDW, you can have a blast on the things he/she can.
I have traveled to WDW a lot with my son so I've walked many a mile in your shoes. If you have any other specific questions, please let me know - I'm happy to assist. Wishing you magical memories!